Been to both several times and would definitely recommend Maui - as many say, "Maui, No Ka 'Oi" - Maui is the best! Whale watching is great at the right time (starting Nov I think), the aquarium is awesome, take a submarine trip, get up early and go up to Haleakala for sunrise (Mark Twain said it was the most sublime spectacle he ever saw and the memory of it would stay with him forever); you can bike ride down it, too. Hang on the awesome beaches, Kaanapali is awesome and Lihue is right there with a fun feel, great shopping and beaches in Wailea, and of course the Road to Hana (which is one of those got to do it once - but the 2nd time, we went 45 minutes and got out to walk deeper in an area and saw a falls, then headed back. Get some literature and read up and I think you'll see the "Maui, No Ka 'Oi!!!" Kauai is nice, for sure, but I think especially with 12/15 yo's you'll have a for-sure winner in Maui. We are going back this summer for the 4th time, 2nd with the kids (13, 12 and 10 at trip time) and doing Oahu and Maui. If you get the chance, Oahu for a few days would be great too, beach is different and the Arizona memorial, and surfing on north side, etc. Have fun, you will have a blast no matter what you decide!!!