i think the 310 is a better mx motor since the revs still climb fairly fast and the power isnt too fatiguing. i have heard of pro lever riders getting their butts kicked by 350's. a nice ported 310 will be plenty of power for the track.
Well I disagree totally, the 350PV can have the most bottom end between the two and a very smooth powerband when piped with a Paul Turner "Race 2003" pipe. It is less fatiguing than any other setup because it has so much "craw down " power that is so important in MX (not having to shift as much). I don't think its any faster than a great running 265-310PV but is has more bottom end and is easier to ride when built correctly.
310 or 330, IMO. I had a TC racing 330 open A bike and loved it on the track. IMO, skill level has a lot to do with engine size as well. If you are a novice or amatuer, it might be a handful to have a 330 or 350 motor.
I race competativly using a stock cylinder that is bored .80 over. I have no problem with clearing any jump I want. A 310 will get the job done, and then some. I can't speak as to whether a 350 is a handfull or not.