Depending on where you have to be in DC metro, consider DCA as long as you have to connect. I live/work downtown and will pay whatever it takes to avoid the trek to IAD (far, far, far from everything!). IAD is good for transcons/international travel (ie. the UA option mentioned above). Definitely go with DTW for connecting; if you are delayed on a PM flight from the west coast, you have a much higher chance of spending the night somewhere other than DC if you arrive late into MSP. There is a 9pm DTW>DCA flight that can usually save you if you are late into DTW though. Miss that 6:45 out of MSP and you're toast.