Its a matter of preference. If you do it right the different size wheel makes no difference in how well it fills the wheel well, it should have about the same overall diameter. Otherwise your speedometer will be really screwed. Mine is just about perfect and my speedo is still off by 3 mph.

I think 17s are the best overall for performance, but show type cars go for 18s or 19s more. Which direction are you wanting to go??