I returned last month from my Honeymoon in Hawaii and stayed at the Wailea Marriott for 5 nights. We had a wonderful time, with wonderful service. I can't speak for the RC, which I assume to have outstanding service and amenities as well.

One thing you might want to consider is the location of each hotel in relation to the island. While the RC might be the superior property, Kapalua is not as centrally located on Maui if you are planning on doing activities that are not on the North side. While nothing on Maui is that far apart, it is a good drive to OGG, Hana, and Mt. Haleakala. Waliea puts you a bit closer to these areas and to a great little breakfast place in Kihei (if you don't want to spend the money on breakfast at the hotel).

Just my 2 cents, I don't think you can go wrong with either property.