i currently own a 2003 501.
prior to buying this i was thinking about the FE650 and test rode one (a 2003 model as well)
both bikes are pretty much exactly the same weight.
the power of the 650 was not scary or intimidating, but definitely very fast. hoilding the throttle wide open made the trees whip by real quick! after riding bike for only 5 minutes i was pulling wheelies at 160kph+!!!!!
for me the real difference is in the turning.
when you lay the 650 over and gun it, the bike immediately wants to stand upright and charge in a straight line. it is almost impossible to lay it over and rail a rut like you do on say a 125cc 2-stroke.
it is a result of the longer stroke and heavier crank of the 650 engine creating more inertia, and even tho the bikes weigh the same, they corner very differently.
like others have said, the advantage comes in high speed sections where you can really use all that power, or if you like cruising at relatively high speed for a long way.
you can make the bike corner if you work hard and are strong enough to muscle it around, but your friends on 250's or even 450's will be riding at the same pace throught the tight stuff with half the effort.
keep in mind my 501 pulls about 170kmh(i think about 105mph) WITH STANDARD GEARING!!
the question is, how fast do you want to go???????
another angle to keep in mind that one of these bikes (650) will possibly last longer, since you won't be wringing it's neck like you might a 450.
the other thing i have always found is that overtaking your buddies while on the back wheel really stirs 'em up, no matter how fast they are through the tight stuff. my rule of thumb is look at your mates bikes and go one bigger just to psyche 'em out!!!(because we all know that bigger is better......right???!!!!!? :? )
Have fun