I fly NW/DL since 90%+ of the flights from my home airport (MSP) are NW/DL. And in the few cases when there is more than one carrier with a direct flight to where I am going, I generally stick with NW/DL, since if I nickel and dime my way across multiple airlines, I would end up with less FF miles (and status) on NW and nothing very useful on the other airlines.