@MXX: It all depends what you are hoping to do, but summer isn't the best time to be in Hawaii to be honest. Either way, the best way to make the right choice is to look at it like this... Maui is basically a cross between Oahu and Kauai; while
Kauai is a cross between Maui and Hawaii.
If I had to choose I'd choose Maui because the restaurants and nightlife is better, and the whale watching is amazing! I remember seeing the whales jumping out the water from our kayak rental in Kahana.
Also, the beaches and drives on Maui around both ends are beautiful, though the beaches in Kauai are miuch better than Maui in my opinion.