Screen size matters.
I definitely prefer the 12.9" pro compared to anything smaller.
Edit: we have the sizes 7.9, 9.7 and 12.9 in the family…
I own neither iPad, but I do have the larger (12.9 inch) iPad Pro. So, obviously, discount my advice accordingly.
Generally, in my opinion where writing is concerned, bigger is better (and the 10.5 is I believe one-fifth bigger than the ordinary iPad). I find that seeing as much of the page as possible when I’m writing is helpful. Your age may also indicate that bigger is better: old eyes can deteriorate surprisingly quickly.
But if you need mobility for your iPad then obviously 9.7 may be a better size. For myself I’ve not found the 12.9 difficult to travel with, and so the same must be true for the 10.5 (and unlike the 12.9, the 10.5 is, I’m told, supportable by one hand for long periods).
I can’t obviously comment on other factors such as power and screen resolution, except to say that I believe that sheer grunt is less important for Scrivener for iOS than for some other iOS applications.
If you can, go and try each model out at your local Apple store, if you have one. Try out their virtual keyboards. That’s what I did before I settled for the 12.9.
Thanks very much for that. Really well considered responses and very helpful! The thing I forgot to mention was that I spend 90% of my time in front of an iMac at my desk. The idea is to have something portable and handy when I have to travel for meetings. There might be some writing but mainly note taking and such. Anyway, thanks again, really helpful!
I use the 2018 version fine for writing. I use the on screen keyboard for short pieces and a bluetooth keyboard when writing longer ones. Mind you, the 9.7" screen is the biggest tablet screen I’ve had! Hope that helps.
I have an iPad Mini with an external keyboard and love it. The combined unit is about the size of a “junior” size paper notebook, making it enormously more portable than its bigger cousins. I’d definitely recommend thinking carefully about your actual usage before buying.
Katherine
I have an iPad air 2, but hardly ever use it for Scrivener any more. My Mac is a Macbook Air 11, and whenever I deliberately go out to write, I pack it. For ideas that hit while I’m standing in line at Costco, my iPhone 6s Plus works fine. I keep a folding keyboard for it in my purse, so if I find myself at loose ends (long wait at doctor’s office, etc.) I can pound out some words. The iPad is relegated to project planning stages (crude map drawings, handwritten brainstorming, etc.) which get sucked in to Evernote with the rest of my research.
I therefore find myself agreeing with Katherine that agility can be more important than screen area. Were I faced with replacing the iPad today, I certainly wouldn’t go bigger than the iPad with Apple pencil, would consider the iPad mini, and might not replace at all.
My usage is similar to yours: most Scrivening is done on a Mac with Apple Cinema Display. My 9.7” iPad Pro is used when I’m at libraries, coffee shops, etc. I find the screen just fine for using Scrivener, though I’m sure I’d enjoy it even more at 10.5” or 12.9”. But since my iPad, like yours, is mostly used for note-taking and occasional mobile Scrivening, I wouldn’t want to sacrifice the portability (nor the dollars) for a larger screen. If I were a full- or most-time iPad Scrivener, I’d want a bigger screen.
I do use a thin, light, i nexpensive bluetooth keyboard with my iPad, which of course leaves the entire screen free for Scrivening, and I’d highly recommend getting one. I have to charge mine about once a year, and it’s as simple as plugging it into a USB charger. I haven’t used the Apple Smart Keyboard because of decidedly mixed reviews and high price. So for me, that Smart Connector capability is another advantage of the iPad Pro that wouldn’t really benefit you unless you just love the ASK.
That leaves the screen quality, and I have to say that the Pro’s TrueTone capability really is, for me, a significant advantage over the regular iPad — but that’s because I spend several hours a day reading, not Scrivening, on the iPad Pro. For short Scrivening stints, I doubt it’d make that much difference.
So based on your anticipated usage, I’d suggest getting the regular iPad and a good bluetooth keyboard and save a lot of money. Let us know what you decide.
Some very insightful responses to your question, let me add another view.
I bought the iPad Pro 10.5 specially for using Scrivener while on the road. I find it to be the perfect companion for frequent air travel; the screen resolution, size, and functionality of Scrivener iOS are a wonderful combination for traveling writers. I added the Zagg rugged case to protect against the inevitable knocks and drops common to air travel and, again, am quite pleased with this accessory.
The keyboard is bluetooth, not a detractor for my use, and has an acceptable feel and key spacing, Both the iPad and keyboard/case recharge separately and the battery life on the keyboard is excellent (a couple of weeks at my usage rate). The support stand on the case is stable and adapts to even the most meager of airline tray tables.
Good luck. 
I’ve got that same Zagg Rugged Folio case and find it quite handy with my 2018 iPad. The keyboard is lighted, which helps in darker situations. It has the magnets to turn on and off with opened and closed. The keyboard syncs faithfully, something not true of all BT keyboards. I like how easily the iPad section splits off the keyboard when I need something light for just reading. The only downside is that it weighs much more, but that comes with protection.
I picked up the version for the iPad Air since it is the same size. The only glitch is are the volume up/down keys are offset slightly. The up volume button now lowers the volume and for up volume, I reach inside the opening for a button not on the new iPad.
All in all, I quite happy with using it as a laptop replacement at less than half the price. I would, however, strongly recommend getting the 128GB model. Eventually 32GB is likely to prove to be too little.
I’ve yet to get the Apple Pencil, but I have begun to wonder how Scrivener could take advantage of it. Perhaps different orientations or pressures could be use for different tasks when editing.
Speaking of Zagg, this looks very promising. I’ve seen some reports of quality problems in Amazon Zagg reviews so might want to wait till after it’s been tested, but it solves one of the problems that my current set up (Apple Wireless Keyboard + Incase Origami) also solves: you don’t have to buy a new keyboard every time you replace your iPad. And it works with my iPhone too.
For me, it’s still hard to decide which iPad is better to get. The iPad Pro (12.9-inch) in the Apple iPad generations list seems quite good to me. Cause it has a bigger screen and support to use a smart keyboard and apple pencil.
Another vote for the 12.9"! I love it. Can’t read with one hand, but having owned a 9.7" that wasn’t really tenable anyway. (And I have big hands!)
Plus, the full size smart keyboard is great. Some people don’t like the feel of the case, but never having to worry about charging it, or bluetooth connectivity, or getting a separate stand/cover, is more than worth the price of admission.
Also, mine failed a few months ago and Apple replaced it, no questions asked. It was about a year and a half old, and out of warranty.
I’ve been using iPad 2018 and it’s just perfect for me. I don’t need all the power and better resolution of iPad Pro because I only use iPad 2018 for writing and listening Spotify at the same time. I’m using Apple Wireless Keyboard (2011) as an external keyboard and Targus VersaVu Case and with those my writing experience is perfect. Apple’s keyboard feels just like writing with laptop’s keyboard, but it’s small enough to take it with me while going to cafe or library etc. Can’t be more happier with this combo!
I’ve just received the new series 3 iPad Pro 12.9 and IMHO it is outstanding. I’ve had and loved the previous 2 12.9 models, but this one is so much more comfortable to work with. Size and weight about perfect and the new 2 position folio keyboard in much better for typing while lying on the bed,
Just got ipad pro 12.9 with keyboard and pencil for partner wanted two but only one in stock so I have been playing with it and shared the scrivener app to it to play with until I get mine. Was still toying with 11 for portability but this 12.9 is so light weight and a thing of beauty and scrivener works lovely on it not tried pencil with that app yet but new pencil is neat tap pencil and it changes nibs or colour depending on how you configure it
Got the third gen 12.9 iPad Pro yesterday, with the new keyboard, it is lush. I don’t know if there is much need for it for scrivener. But I don’t drink or smoke and that’s how I justify the stupid price. Seeing how text looks when you scroll on it is beautiful;.Now I’m googling whether there is a split at selection on iOS scrivener or ill have to ditch the shiny new toy for my laptop.
I have been using Scrivener on my iPad Pro with no issues; works pretty good, you may also give it a try and then decide whether to go for any other device or stay with the one you are talking about. newpipe
Split at selection == cmd-k. Or, long press at the selection and pick “split” from the popup black menu.
And now with the 2018 iPad down to $230 on sale, and a much recommended Logitech or other bluetooth keyboard for under $30, and a good cheap case and/or stand you can have an excellent Scrivening set up for under $300. That’s pretty amazing!
I took a look at the new iPad Pros at my local big box store last week, and while they’re gorgeous, for the kind of writing I do in Scrivener or other apps, they’re not worth four or five times the price. I’m sure they are worth it for others, particularly those for whom the iPad is their only computer and those who do more than read, write, browse and email with it. But at those prices, I wasn’t tempted to upgrade my 9.7” 2016 iPad Pro, which except for the display is pretty equivalent to the 2018 regular iPad.
I often prefer to write, even in Scrivener, on it and my old Apple Wireless Keyboard, rather than my Macbook and Cinema Display. iOS has almost evolved to the point where I could imagine doing all my Scrivening on this little iPad, but for a few annoyances like the inability to import multiple files simultaneously into a Scrivener project on iPad. I’m considering bringing only my iPad, keyboard, and origami case/stand with me on holiday, even though I’ll be writing and posting several stories.
In fact, if I were advising anyone on a budget, like a college student, on what computer to buy for the above tasks (which is what most people use computers for, I imagine), I’d probably just say get a new regular iPad, keyboard and case and call it good. I know Apple’s getting a lot of grief for its recent price inflation, but really, for what most people need from a computer, and even for much Scrivening, the regular iPad seems like a very good deal. I’d be curious to know if anyone here is using it for all or most of their Scrivening, and how they like it, in particular how the screen treats the eyes in long-term use. William (OP), what did you end up getting?
I’m using iPad 2018, Apple Wireless Keyboard and Targus VersaVu case. Those are the perfect combination for me. I don’t need iPad Pro for the same reasons as yours. I use my iPad only for writing. The screen is perfect for my eyes, no issues so far after several months of writing.