I have the 10.5 256 GB because I wanted to carry it around in my backpack and pull it out and draw when hanging out at coffee shops or at a friends house. But if you are planning on doing most of your work in your studio or at home, I would go with the 12.9. Also another thing when considering what size to purchase, the 12.9 doesn't take video in HD like the smaller Ipad Pros, not sure why.
Sure the 12.9 iPad Pro takes HD video. 720p hd @ 30 FPS, 1080p hd @ 30 or 60 FPS, 4K video @ 30 FPS. These specs are right from Apples website.