CLT

CLT has a lot better weather year-round than PHL ( and better OTP ). If you're after the numbers, I believe you can find OTP on flightstats and in DOT reports.

CLT is also easier to tranfer due to more convenient layout.

An alternative would be LHR-IAH-SAT on UA. Also, check fares to AUS - it's a relatively short commute to SAT from AUS by a rental car.