NW has more options for earning miles with partners to top off an account. DL has had a number of promotions that also included MQM earning with partners recently, but those are all gone now. NW/CO have reciprocol upgrade benefits. And then there are the partial credit issues where cheap fares on DL/NW earn partial on CO and cheap fares on CO earn partial on NW. I'd pick NW lacking any other motivating factor. Of course, the best bet is to earn on the carrier you fly the most.