I would choose first based on the most convenient DC area airport...if you are working/living near BWI, it makes zero sense to schlep all the way out to IAD to fly a specific airline/product. If you are in downtown DC, I'd bias myself toward DCA.

Next, I would choose based on schedule and/or product and nonstop availability. For instance, if flying from BWI, I would choose AS for BWI-SAN over a one-stop on anyone else at least westbound. Eastbound, maybe not, because SAN-BWI is a redeye and you could probably get a better one-stop non-redeye on someone else.

Consider buying a few different one-way tickets on your first couple of round trips and trying out different products and connections?