It is a personal choice. It sounds like you have never been there before. Thus, Oahu may be ok for 6 days. Many people see Oahu once or twice and tend to go to Maui, Kauai, or the big island thereafter.

Oahu is great for people watching on Waikiki Beach and the International Marketplace!!! The Royal Hawaiian Hotel is fabulous for at least a few days. It is well worth seeing the Pearl Harbor memorials for the USS Arizona and the Missouri. The big waves on the north shore are cool. Punchbowl cemetery is interesting and gives a great view of the city.

BUT Maui is hard to beat for relaxation, beach bumin', and golfing. Scuba diving and mere snorkiling are very good. There are many excellent resorts on the north and south portions of the west side of the island. Lahaina is a touristy area worth seeing. Seeing Haleakala is fantastic at sunrise on a clear day, and is well worth it later in the day too. The slow drive to Hana is great for some, and a bore to others. The boat ride to and from Lanai [and maybe a night or two there] is fun.

What else are you interested in?