I've been to the Big Island and Maui - I love Big Island MUCH, MUCH more. Maui is busy and crowded in places - the road to Hana sucks bc of traffic.
I would HIGHLY recommend the Big Island - less crowded, gorgeous beaches, amazing lava, less "developed", the road to Hilo is worth it, Kona is neat, horse riding in the Waimea (sp?) valley was beyond. We stayed here, at the Mauna Kea:
http://princeresortshawaii.com/mauna...ort-hawaii.php. Very private, amazing snorkeling. Staying in Kona would probably be fun too - plenty of condos there, which might be more affordable than a hotel (food).