I am upgrading from an Air 1, so it will be a pretty big difference, I would imagine. Usage is probably 60-70 percent or more during commuting for web, email, video/book viewing, listening to music, notes, etc. However, I also use the iPad for sheet music and would like to use it while on vacations for photo transfer from my camera and basic Lightroom Mobile editing (I have an iMac for heavy lifting at home). I'm thinking that the 10.5 is probably best for me for portability and a size compromise although I imagine the 12.9 would be nice for larger sheet music and extra real estate for image editing. So, I guess I am leaning toward the 10.5, but wonder if most would agree.