I'm a rathr small person (5' 10" but I only weigh 122 lbs after all this cancer stuff over the past year. Even when I was my normal 140 lbs I don't know if I'd want the extra weight of the 650, I also like the higher redline on the 400 because if you go to rapping it out it sounds meaner.
Parts are easier to find for the 500 as well.