I haven't flown for either, so take it with the appropriate amount of salt, but I've known people who have flown for both. AFAICT, the QOL at Wheels Up will be significantly better. You will upgrade more quickly, and I think the money might be slightly better as well (for the first few years).
OTOH, Southern will be vastly better as an experience and or resume builder if your goal is the airlines. Also, if you wind up sticking around, Southern will (probably, someday) be a better "career" job as you get higher in seniority.