I'd probably do the opposite of you there :P
If you go often, I would have 18s as the daily driver and another set of wheels purely for track use. 16's or 17's if you don't go often and just use them on the track as well, once in a blue moon.
The question I'd be asking myself is if you lean more towards 'looks' or more towards 'performance'. What future plans do you have for the car? What kind of car do you want it to become... etc,etc.
Future plans...
i dont think 18's could get u enough traction... 18s suck!