Hey Padraic,
Four strokes are easier to ride, because of their ability to make more power through out the entire RPM range instead of only when the powerband hits like with a 2 stroke, however when they break, they're very expensive to fix. If you have the money they're the only way to go though. A two stroke 500 is a heavy bike and can be a pain in the ass to kick start, since its so big. 500's are good for enduros, trail racing and things that dont require the rider to continually change the position of the bike underneath them, like when riding a track. I would say that your best bet is a 250 two stroke if you're going to be doing track riding. It has ample power, is relatively cheap to fix when it breaks, and it maneuverable enough to really get on it at a track.