If you are looking at a FF program for the upgrades, it probably does not matter much, In my experience they are hard to come by to or from the west coast. On any one of these airlines you are probably going to fly from SAN to a hub in the east. CO has a great FC product but relatively small FC cabins. NW and DL have more FC seats per plane but the product is not quite as good as CO. Remember NW and CO will upgrade you on each others flights nd DL will only upgrade you on their own flights. NW or CO do not receive upgrades on DL nor do delta elites receive them on NW/CO.

Other than that probably it is a crap shoot as to which airline you choose. I believe that CO has some funny fare rules where you only 1/2 credit for miles flown, I am not sure about DL.

I'd probably go for DL if you already have 13K in miles this year. They are all about the same.