We loved the big island in November. Was in Maui and Oahu over 30 years ago.
http://forum.calgarypuck.com/showthread.php?t=141031
I'd vote big island just because it is the biggest and has more to see. Besides beaches, snorkelling and surfing, you have the option of seeing volcanos, and the observatories on Mauna Kea. Children under 16 are not allowed to go to the summit.
Stay around Kona. Hilo is on the rainy side of the island. Kona is home to the famous Iron Man race. See where Kona coffee is grown.
Maui thread:
http://forum.calgarypuck.com/showthread.php?t=129062