I have been Gold or higher living in the D.C. area on Delta, US, United, and Northwest. I currently fly almost exclusively out of (actually, into, now that I have moved from D.C. to N.M. ) IAD on UA now, and generally reserve my upgrades for long-haul flights.

You might want to consider NW. As Gold and even Silver you have unlimited upgrades, and actually decent shots at getting them, and a first-class product on the mainline jets that's probably slightly better than United's. They have decent service out of DCA (although it's from the horrible ghetto-terminal A) -- their schedules are probably better than UA's from DCA, for example. Of course, you lose most non-stops to the west (from IAD on UA) or to the south (from DCA on US or DL), so NW would only be viable if you don't mind flying mostly through DTW and MSP, which are, in my opinion, two of the best airports in the U.S. through which to connect. And WorldPerks is a decent program, with pretty good SkyTeam coverage out of IAD if you want to go to Europe.

I can't speak to Continental at all, but I had very poor experiences upgrading as Silver and Gold on Delta.

I can't believe that I am recommending Northwest Airlines, which generally has poor service all-around, but for a semi-frequent flier living in D.C. it's an OK choice all things considered.