I wear 9.5, and for longterm wearing I prefer 9s, I have both 9s and 10s. The 9s are tight to begin with but they will stretch and be perfect after a few months.
As for rubbing between the toes, fitflops do need to be broken in a bit. I am on my third pair, and the first week I always break them in, and get my inner toes used to a new pair. I can now wear my new ones everywhere..I was trekking around campus yesterday no problem!
I love mine, I also have back problems (I had surgery and have screws and bars in my back) and I NEVER have back problems wearing these.