Maui is very, very touristy. If you go to Maui, and night life is your thing, the only area really open is the old town of Wailea. If you spend more than a few days on Maui you will find yourself driving quite a bit to Wailea.
Forget the "big island", it's boring and not very pretty.
For my money, I would go to Kuai - the garden island. Lot of small, hidden beaches everywhere. The guide book(s) to Kuai are worth the money. Get you to places off the beaten path. But, not a lot to do in Kuai after sun goes down, besides the luaus.
Don't forget no matter when or where you go, the sea water is much colder than you think / expect. If you or yours can't stand cold, you might think about full or at least half wetsuits. You can't go to hawaii and not plan on at least snorkling. The locals might frown on this - put sneak a little seafood-based dry catfood with you if you snorkle. you will see lots and lots of fish in a frenzy.
best of luck - Tim