I have a 1978 xs750 and a 1979 xs650.
The two bikes couldn't be more different.
The 750 does have more power, but I still wouldn't call it fast. The 750 is a heavier and longer bike, and I think it is best suited for highway cruising. The triple motor and shaft drive are SO SMOOTH, you just glide along. It feels like a big pig in stop and go city driving though, the extra wheelbase length and weight are very apparent at lower speeds. Get a 1978 or newer 750 because they have CDI ignitions, you don't want to mess with 3 sets of points... they make the 650 dual setup seem like a breeze.
There is very little aftermarket or parts support for the 750, so eventually you'll end up needing to buy a 'parts bike' too, since its by far the easiest and cheapest way to get spares for the 750. don't buy a beater 750, you'll pay for it in repair bills over and over again. Buy the nicest one you can find, or don't buy one at all.
I very rarely ride the 750. My xt500 and xs650 are both more fun, lighter, and handle better. The 750 is better on the interstate, but I prefer my Harley Superglide for all day cruising... so I'm not really sure where the 750 fits in. I only paid $500 for it, and its pristine, so I keep it mostly as garage decoration (its NOT the Special model, its the pretty one with the factory café-ish tail and gas tank with knee indents).
Personally, I think the 750 Specials are a waste of time, fat, ugly, and not fast enough. The only real saving grace of the 750 is how smooth the ride is and how good the 'sport' / 'cafe' model looks.
Here's my 1978 xs750 Sport: