CONUS only aircraft @ UA, AA, Delta
At AA, your bid preference is 4-part, base/position/aircraft/domestic or international (e.g. LAX/FO/737/D). So, here you'd eventually have a choice of S80 (retiring ~ 2018), A320, 737, and 767.
You wouldn't want to bid 777, 787, or any international preferences with the other airframes listed above.