I have been to both, and spent some time in each. I lived near Nashville for a while.
I think that Nashville might be a better bet because it is a newer city and its people are more cosmopolitan (and thats a stretch).
STL is NOT a good idea. St. Louis is a run down dying city, and its VERY conservative. By that I mean that they are just like Memphis, over a million people, but no one travels. Its just churches and gas stations. Talk about seeing the world and experience life, and they hit ya over the head with a bible and try to "save you". There is not enough originating traffic in STL, just like Memphis. Yes, NWA has a mini hub there but it is strictly to move people from one place to another through there. If there was enough traffic, AA would not have left STL to begin with. And STL sucks! Gates are run down, JUST LIKE THE CITY.
I think BNA, but alternately, I really think that culturally, environmentally, population-wise, and economically (weather related costs, labor costs) I'd hedge my bets for San Antonio!

1- Over a million people. WHO travel and appreciate something "different"
2- Revitalized town - along with a revitalized culture and attitude.
3- Weather related costs would be low - no snow, or fog. Watch out for thunderstorms.
4- Major military installations = lots of traveling soldiers.
5- Ethnic diversity = people who want to visit their origins, their families come visit.
6- Facilities. I've been there, the terminal is really nice, new, and BIG!
7- Resources - San Antonio Aerospace is there and could be contracted to provide maintenance so you dont have to be held captive by your own workforce or spread the work between both (remember SWA has its own MX, but it contracts out its very heavy MX C and D checks to the lowest bidder)
8- Labor costs. It is the South after all, and they are pretty much anti union; save for the pilots of course. But if youre looking at using EMB-190's youre already looking at lower-time entry level pilots anyway.

My $.02