I probably shouldn't disagree with someone who has "Weather" in his username, but I find Kapalua to be quite a bit more likely to be rainy than Wailea, based on dozens if not hundreds of visits to each.

Not talking the type of all-day soakers brought on by Kona storms or cold fronts, which would impact both locations, but just the pesky sprinkles blown over by trade winds that can dampen an afternoon at the beach/pool or an outdoor meal.

As others have said, you can't go wrong in either place. But Wailea, and especially the Andaz near the Northern edge of Wailea, should keep you mostly dry outside of a significant weather event.