You have to consider how much you want to change flights and whether that airport hub is good or not as it may add considerably more travel time. Say if you choose CO, you'll be flying through EWR (delay prone) or IAH a lot, but with CO you could also fly with NW (hub at DTW, MSP and MEM) and DL (ATL, SLC and CVG). With UA, the hubs are IAD, ORD and SFO but you could fly US with hubs in PHL and CLT.

I'd say AA would be a poor choice if you had to fly to the NE U.S. (almost all their flights would hub through ORD or DFW)