The conveniences and elite privileges of an alliance are really secondary to those of your "native" program, and there is arguably as much variation within alliances as between them— you're more likely to find a flight on *A Singapore much more comparable to OW Cathay Pacific than to OW airBerlin. And, as with your home airline, the "best" alliance depends on your travel patterns and priorities.

It's still far too early to tell what will happen with the merger. We don't know what the FF program will look like, or what they will inherit from legacy AAdvantage or DM. US currently has the most departures out of ITH, but they might cut some of them, making DL or UA more competitive. They might decide to route ITH flights through JFK instead of PHL. They might switch Express contractors, or replace them with Eagle, and they might change the equipment. I would look at all of these things before thinking about the impact of an alliance change, especially since you say most of your travel is domestic aside from the UK (where AA/BA offer the most seats and flights). It will be months before it all shakes out.