I think of 7.5's as novelty guns. Absolutely nothing wrong with that, just limited in their ability for longer range shooting. Most say that about 10.5's as well, but they're awesome imo. Easy to move around with, quick target transitions, and on a single point sling, they don't hit the ground unless you do.
10.5" is perfect for the style of shooting I like. I'd get a 7.5" one day, but probably not before a 14.5/16" and not as my only AR.
Do you have an AR already, and if yes, what length barrel?
Also, it's your cash, buy whatever floats your boat!