As you mentioned, each island has its own pros and cons. We've been to the four main islands and our favorite is Maui. There is a great beach right in front of the Westin Villas (KORV) that has easy snorkeling, in addition to the Black Rock outside of the the Sheraton close-by and further to the south. One of our friends used to swim from Lahaina to the KORV and back every morning for his daily exercise! There are many restaurants still available for nice dinners out. Tours to Hana and Haleakala are available for those who are hesitant to make the long drives. Helicopter flights are a special way to see the island. There are lots of areas for hiking. The Westin villas on Maui have all of the other amenities that would make your trip a memorable one (pool, spa, and sunset sail availabilities). For shopping trips, you would probably need to drive back into Kahului or down to Wailea.
We also enjoy our visits to Oahu, especially for the shopping, the abundance of eateries, and the chance to visit with old friends. However, coming from a busy urban setting, we find that Maui is a more relaxing vacation for us. Our friends on Oahu are avid hikers, so there are lots of trails that they enjoy.
I'm sure that your friends will help you with the choosing which island to visit, but I am sure that you will enjoy the time with your friends on any of the islands! CJ