I was born and raised on Kauai, and as much as I love Kauai and think it's a fabulous vacation spot, for teenagers I think MAUI is a far better choice.
Kauai is incredibly boring unless you enjoy the typical Hawaiian outdoor activities (which I did not). Being an indoor girl, aside from my school activities, Kauai had very little appeal to me.
Oahu has great everything, but it is so crowded that I can't stand Oahu. I think Maui is a perfect blend of the beauty of Kauai, but with a little more bustle and activity like Oahu. Not as much as Oahu, but a good amount.
The food in Hawaii seems the same wherever you go, although Kauai has Hamura Saimin which people seem to love love love. I don't get it, it's just noodles to me, but I have met tourists who will go straight from the airport to Hamura Saimin because they love it that much. So Kauai has that, but I am sure Maui has something similar, a restaurant that locals will swear is the place to go. Oh and Maui has guri guri ice cream so that's another plus for Maui.
Maui will have better shopping than Kauai, but the best shopping is on Oahu. And I am sure by this time all the islands have ziplining. And they will all have luaus.
So Maui I think is a better choice, and I like Kahului on Maui.