It seems to me that you are going to have many connecting flights and few point-to-point nonstops. If you're flying to Dallas from DCA, AA has by far the most flights (US has only a 3 daily, all in E170's which don't have a First Class section. If you're going to the West Coast, on AA you'll end up changing in either DFW or ORD (maybe JFK - but that's a Jungle Jet connection). Most flights I've seen ex-DCA are pretty full, so I am not sure the odds of an upgrade on US or DL are that much greater than on AA.

Also, DL and US just did a route/slot swap deal and over the next year you'll see fewer Delta flights and more US flights at DCA, so I don't think Delta would ba any improvement; also, on US, it depends where they go, and if they fly mainline or regional jets. Connections to the West Coast would mean a change in PHL or maybe PHX instead of ORD or DFW on AA, and PHL is not a particularly nice airport to connect in IMHO.

If you're generally happy with AA and occasionally do get an upgrade, stay with AA. If you want to try US (which is, again IMHO only, an inferior service level most of the time, an inferior product to AA), then maybe ask for a status match to see what happens on a trip or two.

Jeff