I think you just need to ask yourself more about your particular situation and preference. It sounds like you take your reader with you quite often. In that case, will carrying the larger Forma be a hassle? Or at times even impossible?
I purposely avoid the larger screens. I do read at home a lot, but I take my reader with me every time I leave the house, even if just to the grocery store now. The time my car broke down and I had to wait 3 hours for the tow truck it saved my sanity. I would have been going out of my mind sitting and doing nothing for 3 hours, instead it was very enjoyable on a nice summer day. At least until the tow truck driver insisted on talking on the drive back!
So for me, the portability is much more important than a larger screen, and I don't care for physical page buttons. That's what you need to figure out for yourself, what's most important to your situation.