I have the 10.5" iPad Pro 2017 and I have tested it side-by-side against the 11" iPad Pro 2018, and my friend has the 12.9" iPad Pro 2018.
For regular simple usage, the 10.5" is more than sufficient in terms of speed, as it is extremely fast. However, there are very rare stutters in the UI that most people wouldn't notice, but if you have the 11" beside you you can occasionally notice on the 10.5" because they are not there on the 11". So, that is to say that the 11" is perfectly smooth in the US, the 10.5" is near-perfectly smooth.
For surfing it's great, but the one thing I've noticed is that the extra screen width of the 11" matters, vs the 10.5". The extra 0.5" actually helps, not just with single window browsing, but it also provides a nicer Split View experience with other apps. However, that's irrelevant for since you're interested in the 12.9" model which has the same screen size for the 2017 and 2018 models.
One thing I've noticed where lag/performance can matter however is in video editing and photo editing apps. I couldn't directly compare this on the 2018 iPad Pros because they didn't have the same apps loaded, but I definitely notice the lag in certain actions on my 2017, whereas other people say that that 2018 models often have much less lag.
OTOH, video exports from LumaFusion on my 10.5" Pro can be very fast. It seems the hardware encoder in the A10X chip is already very good. Below are some tests I did, for HEVC 4K 8-bit transcodes.
https://forums.macrumors.com/threads/hevc-encoding-2017-12-macbook.2178209/#post-27294212
I don’t have Compressor or Adobe Media Encoder, so I just used QuickTime.
Sony Nature Camp 2’07” video - 76 Mbps 4K 10-bit 59.94 fps
MacBook (QuickTime 10.5 Mojave HEVC export): Forever. Not feasible. Probably was 10-bit encode (software)
iPad Pro (LumaFusion 1.7.7 HEVC 30 fps export): About 2 minutes, converted to 8-bit I believe.
Sony Nature Camp 2’07” video - 79 Mbps 4K 8-bit 59.94 fps
2017 MacBook Core m3-7Y32 (QuickTime 10.5 Mojave HEVC export): 8’37”
2017 iMac Core i5-7600 (QuickTime 10.5 Mojave HEVC export): 4’52”
2017 iPad Pro A10X (LumaFusion 1.7.7 HEVC 59.94 fps export): 3’51”
2017 iPad Pro A10X (LumaFusion 1.7.7 HEVC 30 fps export): 2’03”
---
That's performance. What about other factors?
The accessories here for the 11" iPad Pro cost a lot more than the accessories for the 10.5" Pro, and there are none of the good sales the people in the US get. That, and the decent refurb pricing on the 10.5", made a big impact on the decision for me to get the 10.5". Pencil for the 10.5" is cheaper, and for some reason Apple Smart Keyboard is available in Canada occasionally at a deep discount, but the same is not true for the 11" model.
Pencil 2 on the 11" Pro / 12.9" 2018 Pro is much less awkward because it magnetically attaches and charges on the iPad Pro. Plus the double-tap really is a nice feature. Otherwise, it feels exactly like the Pencil on the 10.5" (except that the original Pencil is all round and will roll off the table).
The Apple Smart Keyboard on the 2017 models do not protect the back, so I bought a back case for it. This actually makes buying sleeves for the 2017 models harder, because a lot of the tight fit sleeves don't fit well with added back covers. The Apple Smart Keyboard on the 2018 models protects the back (but not the sides). There are also two different stand angles on the 2018 vs. the 1 on the 2017. OTOH, for the 2017 model, when you fold the keyboard back, you get the flat cover. On the 2018 model when you fold the keyboard back, you get exposed keys which feels weird. The solution though is just to take the cover off.
On the 2018 model, if you double tap the keyboard when the screen is off, it takes you to the home screen, after having invoked Face ID. On the 2017 model, if you double tap the keyboard you get to the lock screen, and have to reach out and invoke Touch ID. So, what I do is don't bother with double tap. Instead I just immediately reach out for Touch ID and press the home button to get taken directly to the home screen.
The 2017 models support USB 3, but require a dongle, and don't provide much power natively. To power a lot of external peripherals, you need to plug in an external power source. (The Apple USB 3 dongle has a Lightning port input for power.) The 2018 models have native USB-C and also can provide more power out the port natively. The 2018 models can also charge external devices like your iPhone.
On the 2018 models when I'm holding them in landscape mode, my hand covers the camera and Face ID sensor, and covers part of the edge of the screen. On the 2017 models, there is no such problem.
If it hadn't cost $500+ more Canadian to get the 11" iPad Pro and its accessories, I would have bought the 11". However, in the US, there are some decent deals on the 2018 models at Amazon and Best Buy. In that context, and considering you're wanting to get into video and photo editing, I'd recommend you go with the 2018 iPad Pro.
---
The one other thing that annoys me about the 2018 models' pricing is their tiered memory, despite the fact all of them are already pricey. All of the 2017 models are 4 GB RAM, but only the 1 TB 2018 model gets 6 GB RAM. My next iPad Pro will likely be an 11" 6 GB model with hopefully 128 GB base storage. That's not available now, but I am happy to wait a few years to get that, since my 64 GB 10.5" is sufficient for me for now.
