I think the Four Seasons as a resort overall is much nicer. I went scuba diving there and hung out by the pool for an afternoon, and the setting and service appeared to be divine. But I didn't see the spa (past the lobby, that is.)

The Grand Wailea was a much larger complex with lots more people, particularly kids. It reminded me a little of Disneyland, esp. with the water slides and all the pools. But the spa was a great value, and I didn't see anything like the Terme baths offered anywhere else.