Dear Mr. Watt,
A behemoth United would, in my view, probably keep both new United Boeing planes and new US Airways Airbus planes, for political reasons, at least for awhile. You probably know that Star partner Lufthansa has an open 50-50 buying policy regards the two manufacturers, and does so profitably.
Also, United is trying to please everyone under the sun to get this buyout approved (with insubstantial measures that should fool no one, btw) and the EU has expressed reservations.
In any case, as one forum discussant noted, the two carriers use different engines on their common model types, which will cause financial indigestion in any case.
Need a new airline paint scheme? Better call Saul! (Bass that is)