I checked on flights PIT-AUS for random dates next month. There are no non-stops, and the best fares for one-stop travel were on UA via Houston. USAir was notably higher in price.

US is the chief airline at PIT, but at least for now you can get credit on UA's Mileage Plus for US flights (as I do myself). My personal experience with award availability using UA miles has been quite good. In addition, UA's website is better than most in providing detailed information about award availability on its airline partners and in enabling booking award travel on those airlines without needing to phone.

If you have any kind of CC, you can pick up some miles on either UA or AA by registering the card with their dining programs (one program per card) and establishing a profile.

As far as travel to DC (where I live) is concerned, you may be better off driving or taking Amtrak. Flights won't save much time, especially if you need to go to BWI or Dulles to get a decent fare.