I have stayed at the Four Seasons but not the St. Regis, although I have been to the bar and beach restaurant at the St. Regis several times, as it is close to our house in Punta Mita. My choice would be the Four Seasons, although I think both have their benefits and drawbacks.

The St. Regis is obviously newer and has a more updated look - the beach club has a chic modern look that overall seems more "adult" oriented. In the Starwood forum there have been reports of spotty service and other issues. The hotel is not on the best beach in Punta Mita, but it is close to the golf club and has some pretty good views. I note that there seem to be rooms directly adjacent to the pool/bar, which I think you would want to avoid.

The Four Seasons was the first part of the Punta Mita development and as a result they have the best location, the best views and the best beach. The hotel is huge and sprawling, but it is a Four Seasons and it is impossible to fault the rooms, the amenities or the service. It is also possible to have your privacy at the hotel - there is a more intimate adults only pool and one of the beach areas (the one in front of the beach front suites) is fairly low key. The ocean front rooms are also fairly private.

Note that the hotels are not closed to non-guests. If you stay at one resort, you can easily eat at the other hotel (they are 5 minutes apart and the Four Seasons will drive you around Punta Mita; I don't know if the St. Regis has shuttle service). There are also a few nice restaurants at the private beach clubs for residents of Punta Mita.