Like Eldar said, rider size is a key issue. I could ride a 400f, but I might not be able to straighten my legs out again once I stopped. I'm 6'4". My 550 is a little small, but I like it for that reason.
Also of key interest: how important is looking good and impressing people? You'll find most people agree that the 750s are much uglier and not nearly as impressive as a fine 550. I used to have a friend with a 750... well, maybe friend is too strong a word... He was richer, better looking, and much nicer than I, but, without fail, every time the ladies compared his motorcycle to mine.
My advice, if you got with the 750, get a solo seat. You may need a side-car if you get the 550.