We have stayed in both resorts. As others have stated, they are both fabulous.
We are not golfers but the Kapalua course is (according to friends) something REALLY special. Kapalua is not too far from Lahaina/Kaanapali which has LOTS of activity.
Mauna Lani is quite far (30 miles?) from Kona, so if you like lots of shopping, dining in various restaurants, visiting different bars, it is probably not the best choice, though it has its own dining that is wonderful. The grounds are breathtaking and it is frequented by celebrities (Celine Dion was there just before our last visit).
Both islands have great sightseeing. In fact, we always visit them both because we can never make up our minds. If you are going during whale-watching season (Jan-Apr), it is better on Maui.
Whichever you choose, I hope that you rent a car and have time to do driving around the island. Each has a different feel but each is also "real" Hawaii. Maui is, IMHO, becoming quite commercialized in several areas, Big Island not as much so. Beaches on Maui are legendary, but Big Island has some good ones too.
You will love it. You will surely be hooked and want to go back.