Wailea and Kapalua are both luxury resort areas, but Kapalua is a little more exclusive. Wailea has more activity, restaurants nearby, etc.

What are you looking to do? If you want to relax in splendor, Kapalua might be the thing. If you're looking to shop, dance, and resort-hop, then Wailea is a better call. If you want to play golf, you can't go wrong either way...