doesn't matter too much. The FS of the sub listed is free air, your will be inside a box so the actual FS will shift anyway. Look at your frequency response and excursion/power limits in the enclsoure, that's 99% of it.
Right AT is best, hence why its registered at that free air resonance by manufacturer in the first place; along with VAS and QTS...those are the three parameters you'll need to most effeciently construct