They both only administer USAirways health-care money. The costs are identical. Both have worked flawlessly for some; both have been a royal pain in the a** for others.
The only real difference is which practitioners are in their plan. If you have a list of favorite doctors, pick the plan that includes all or most of them.
When US switched to BC/BS several years back, I lost my primary care physician because he wasn't in the plan. I am delighted that UHC is back because I get my doc back. Others were upset with going to UHC because they lost their favorite providers, so oddly enough, USAirways decided to actually give us a choice. Pick your doctors and decide the plan based on their accepting one or the other. If you have no regular doctor, then it doesn't really matter. If you make no choice, it will default to UHC.
I'll echo what nycbusdriver said. When we had BCBS I had few complaints, and the same has been true for UHC (except for some snags in the change-over from BCBS to UHC under COBRA but all or even most weren't UHC's fault). Our doctors were on both plans so the change from one to the other had little effect, really.