US has the most flights at PIT. However, you should consider the possibility of a US/AA merger, presumably in oneworld, and the mess that the UA/CO merger seems to be. If you're moving to PIT for school (CMU?), does this location have a four year horizon for you?

Note that as a student, your CC opportunities are probably somewhat limited and in any event you should be careful here. In fact, during the academic year, your travel possibilities will be limited. Long weekends in DC and NYC might be incompatible with your studies.

Note that Southwest doesn't belong to any alliance and does not fly to China.