If you fly out of DTW why would you want to go with United and not Northwest? I believe United's flights out of DTW pale in comparison with NW. All your CO miles can be redeemed in NW...I think the chances of United going under or cutting back on DTW flights are way much more than the chances of NW cutting flights out of their premiere hub airport.

You may want to take a look at the flight timetable of both United and NW and compare their flights out of DTW before you make such a drastic move. And get yourself the Starwood AMEX which will allow you to earn a mile per dollar (with a 5k bonus when you transfer 20k points) to pretty much ANY airline.

Just my two cents.