I never had problems with UnitedHealth in the past when they administered the plan. When we switched the entire airline to BCBS for severeal years, I lost my family doctor from the plan and several specialists that I really liked. Personally, I am delighted to get back to UnitedHealth, but I realize that it does not work for everyone.
The best thing to do is to be sure the docs you use are in the plan. If they are in both, then I doubt it makes much difference which one you choose. Perhaps, ask the doctors which plan they prefer to work with.