I've been a few times now and absolutely love Maui... We're going to a friend's wedding in Maui next month and are quite excited!
We both love it there... i think it would be a great spot to get married.
I imagine Oahu might be the most expensive (?) The big Island would be nice... either Hilo or Kona... or the Island of Kuaui... i haven't been there but I've heard its gorgeous!
Congrats, good luck & happy planning!