I have stock 550 and 750. As for parts, haven't had problems finding parts for either, although I believe the 750 should have more parts available because so many more were made.
I first had the 550 and thought it was fine for fit at my 6' height and 33 inseam. But after having the 750 for awhile, it seems a better fit. My knees bend further up on the 550. And wasn't it called Honda's answer the in "inseam challenged" after the 750 became so popular? The 550 is a bit more "responsive" in terms of maneuverability due largely, I think, to lower weight. On a longer ride, I'd always opt for the 750 as it's a smoother ride and as I said, seems a better fit for me.