I have stayed at both the Fairmont and the Ritz. Personally, I absolutely love Wailea, so that gives a huge nod to the Fairmont. However, the Ritz is an absolutely spectacular property, and the spa is phenomenal. My wife and I are returning to Maui this summer, and we are actually staying at the Ritz. This is mainly because of some Ritz Carlton promotional certificates I need to use.
There is no way to go wrong with either choice. In Kapalua, you'll end up spending more time in Lahaina. If you go with the Kea Lani, you'll likely go to dinner in Wailea and Kihei on most nights. Wailea is also closer to Haleakala in case you are interested in that.