There's another factor influencing my choice. Both directions on the Northwest flights are a minimum of 25K miles, so I'll use my entire points balance for this trip. However, if I fly back on Monday, I can fly first class for the same 25K miles. (Oddly enough, on the flight in question, there are no coach seats, just a 25K mile first class seat.) So is it worth an extra $100 worth of hotel room and a vacation day to fly back in first class?
Addendum: The first-class flight requires two plane changes (MSP, SLC), whereas the coach flight is only one change (either DTW or MSP).