There are definite tradeoffs. I am also spread across NW, AS, and CO, with the occaisional KL or AF flight thrown in. For me, NW offers the best FF coverage, but I've been tempted to go with AS when I stop commuting between Seattle on Honolulu later this year. Since we can finally get SWUs on NW, I'll probably stick around, but you might not be logging that many miles.