Kauai: more budget friendly than Maui/less commercial, Aloha beach resort still around $100/night, less traffic, nice waimea canyon area w/lots of hikes, Brick oven pizza, good snorkeling, better hiking than Maui. Hanalei bay good for surf lessons.
Maui: nice, but more traffic, road to Hana (nice to do once), bike down volcano mtn/ride, better for water sports/windsurfing, great snorkeling, molokini crater.
big Island/ Kona: can be traffic congested too near city, but go for the volcano, hike across bottom of crater, doesn't seem to be as many beaches due to lava rock, nice beaches north of airport, can see snow capped mountain while swimming in the surf. liked Sheraton Keauou (sp). can spend a lot of time driving around island to other side.
I don't think you can go wrong on these 3 islands, but Kauai is our favorite.