I was an elite on NW for 4 years, and I spent a lot of my life connecting through DTW. Now, my experience was a bit different, because I very rarely had to fly on an NW Express (well, Airlink) flight, but DTW was a very smooth connection point for me. I very rarely encountered significant delays. If you have to connect from the B/C terminal, you'll get some exercise with every connection -- not unlike UA's T1 Concourses B/C at ORD.

I don't know if you plan on traveling internationally for your vacations, but if you do, the UA SWU's are handy, so I'd make sure you have some of those. (Disclaimer: Despite the fact that I've been around these boards almost as long as you have, it's hard to keep up with the changing nuances of all of these programs.)

IAD isn't a terrible airport for connections. I worked there for a UAX carrier for two years at the beginning of the decade, and our operation was night-and-day better than what it was at ORD (I did some temporary work there, too.)

UA's miles are probably going to be easier to use than DL's, so if you have to pick, that may be one reason to do so.