"What's the difference between the freeze-resistant models and the sanitary freeze-resistant models?"
The sanitary freeze-resistant models have fully sealed assembly minimizing the chance of ground water contamination, and prevents drain water from mixing with fresh water with the freeze-resistant valve designed for ground installation 18" below the frost line. The freeze-resistant models feature only the resistant valve with 18" installation below the frost line. There's also 60" of access tubing provided with the freeze-resistant valve system. The length of the tubing can be reduced in the field for lesser frost depths.