The 750 has more power and the probably made more of them, but it's heavier and doesn't handle as well as the 550 right outta the box. The motor also has to come out of the frame for any top end work on the 750. On the 550 you can do it with the motor still mounted. And top end problems do rear their heads, usually an o-ring at the ends of the head or chain tensioner that won't work.
There also seems to be a lot more changes throughout the 750 model years that make parts non-interchangeable, mostly cosmetic stuff. A 750 expert can chime in here. The 550s seem to interchange pretty well from F and K with a few exceptions.
If you like the 550 enough to consider getting it, I'd check the operation of the cam chain tensioner, pull the tank and sidecovers and do an inspection of the wiring harness to see if it's hacked up or burned at all. Those seem to be the biggies on the 550s.
I'm 5'11" 190 and I fit on the 550 okay, though it was uncomfortable at first because of my clubman bars. I work out a lot more now and it's fine. Shoulder and forearm strength is key here. My brother says I look huge on it. Truthfully I think some true clipons would be best. They're a little higher and usually not as sever an angle as the clubmans. Lots of people do rearsets, just do a search.
