At AA you should be able to get PHl on the 190 or LGA on the 737 or A320. DCA is pretty junior and you should be able to transfer there within the first year. ORD is a bit more senior and would probably take a few years to get. Personally I'd go with United if you plan to stay in DC and really want to fly widebodies. If you don't really care, or think you'll end up in Chicago I'd go with whoever gives you the earliest class date and don't look back.