Agree with previous posters re: int'l at IAD vs domestic at DCA. It also highly depends on where in the metro DC area you relocate to (traffic here can be atrocious). I use DCA (10 mins away) as much as possible coz I live in DC and IAD (40 mins away) is a pain to get to but I do use IAD for trans-Atlantic travel and transcons (regularly). For local/regional and trans-Pacific, I stick to DCA (mostly via ORD on UA). But AFAIK, there are no elite lines for US/UA at either IAD or DCA. US may have a priority line that's open to *A Silver/Gold members at DCA and I do recall seeing it but it's not on their website *shrug*

Besides proximity, I favor DCA because you can get from daily parking to the gates in 10 minutes flat, or slightly more depending on security. Security to get to the UA pier is usually not as bad as the US one but at worse, the security line to the US pier is comparable to an average line at Dulles. At Dulles, security alone would take more than 10 minutes, some days up to an hour. Not to mention the 10 minute walk to get from daily parking to the main terminal. And the 10 minute moonbuggy ride to the gates. And if you have to use the G terminal, well, you'll need divine intervention

Both DCA and IAD have non-stops to IAH. DCA-IAH is on US and IAD-IAH is on UA. They're both E70s so they're much nicer than RJs. But UA E70s have E+ and F ^

I would pick an airport based on regular traffic conditions but your priorities may be different. Good luck!