I've head speculation that IAD would suffer more in a merger, but I just can't imagine that United would let that decline in favor of a greater share at DCA. With PHL so horrific with trans-atlantic ops, I can't imagine they'd keep that over IAD.
However - if you had real professionals from UA running the operation rather than the kids from Tempe, then maybe PHL wouldn't be so bad. Afterall - they run IAD which has many of the same operational challenges as PHL does given they share the same air space etc. And - the terminal facilities at IAD are pretty awful - so PHL could make more sense from a passenger comfort and usability perspective. The UA Express operation at IAD makes terminal F in Philly seem like the best thing since sliced bread.