Why not stay in the Alaska program? You can still earn on AA, CO, DL and NW. Out of BWI, unless you're planning on flying Southwest there is no one clearly dominant airline. Of those four, NW probably helps you the least based on your expected travel patterns (except if routing through MEM is useful), but all four have something to offer.

Southwest will probably have the most nonstops towards wherever you are going, but you'll have to weigh that against how you like Rapid Rewards vs. the alternatives. Southwest does allow for rapid accumulation of credits (particularly if you fly shorter segments and play the car and hotel bonus credit game), but some people don't want to accumulate only domestic awards so they prefer another program.

If you're not going to switch to Southwest, I would definitely consider staying with Alaska since their program is the only real option that crosses airline alliance lines.