DFW Flyer here - I can't stand the dominant carrier AA. Years ago they lost my bags and wouldn't reimburse me for them. Been boycotting them ever since. Considering how restrictive their elite levels are by comparison with NW I can't imagine going back.

Actually I started with CO after AA back when CO offered a good Elite system like NW does now. After CO started the 50% EQM mess though I flipped to NW by leveraging the 2X EQM promo a couple of years back.

Basically for me - as a heavy domestic flyer - its about the upgrades and the Skyteam package. As a Plat upgrades are virtually guaranteed on most NW flights. When NW doesn't work out usually DL or AS works with CO being the last resort. All of them are in the same DFW terminal building. Additionally the lounge system cooperation is also better than most.