The weather differences between summer and winter are very minor. Once, a Maui resident told me how wrong I am about that. He talked about summer heat, trade winds, rain and some other stuff. Perhaps to him there's a big difference.

For me, however, Kihei/Wailea in Jan. (av. high of 81F, 2.6 inches of rain) is not much different than in July (av. high of 88F, 0.5 inches of rain).

As mentioned above, the crowds are bigger in summer, but not in a way that has much impact on your enjoyment of your vacation.