When choosing a program to concentrate on, think about what you want out of an FF program.

If you want awards on lots of different carriers, ditch NW WorldPerks and build up your AS account. MP has tons of partners, and WorldPerks award availability seems to have declined a fair amount.

If you want first class upgrades on transcon trips, concentrate on NW and try to achieve WorldPerks elite status.

Like you, I fly both AS and NW but stick with WorldPerks -- upgrades on short AS west coast flights don't matter to me, but they're nice on long trips to the midwest or east coast, and NW elites get them free every now and then.