I think many of us here would argue that, given their respective financial states, you should do the opposite--focus on UAL and use up the USAir miles. YMMV, but although both are in a pretty tough financial spot, USAir appears much closer to selling assets (like the shuttle routes) and taking other steps that may or may not have them much closer to bankruptcy than UAL is. There are many, many threads on this issue here and more information on some of the other websites I suggested you look at.