La Cantera does have a great spa, but I would stay at the villa properties rather than the main hotel.
Miraval is great. I've been several times and enjoyed it very much. The food is outstanding, and it's included with the room rate. I had a great spa treatment there, but I would say the spa facilities and amenities are limited. Miraval is also very activities-focused. Check out the daily calendar on their website to see if the activities are things you'd be interested in.
If you stay there, opt for a second floor room that overlooks the trees. It will feel like you are sleeping in a treehouse.
Loma de Vida at La Cantera is a better spa experience, if you like to lounge around and use the facilities. I also think the main pool at La Cantera is great, especially since they have an 18+ pool.
I've always wanted to try Lake Austin Spa and Resort, but it's gotten so expensive, I haven't been. It's similar to Miraval in concept, and it is right on the lake (Miraval is not).
I have not been to Trellis at the Houstonian since it was renovated. I stayed at the Houstonian a few years ago, and the rooms were nothing special.
Personally, I think you are likely to have a more private and less crowded experience at Miraval and Lake Austin, since those are more private properties. They also don't allow children.
