If everything else was equal, I would go for DCA, but in your case they are not. Your decision, obviously on the plusses of taking United vs. US Airways.

I wouldn't describe DCA at "tiny" (it's much larger than, say, BUR or even GSP), but it is easier to navigate and to get into DC from than IAD. From DCA you can take the Metro into town, though doing this at rush hour, with bags, can be cumbersome.

From IAD, the aforementioned Metro bus is the cheapest method, but it can take a while, with, at least the last time I used it (a few years ago), no extra space for luggage. I think the next cheapest is to take the Washington Flyer bus to West Falls Church Metro station and then Metro from there. Quckest, and most expensive, options are taxi or rental car. I think Washington Flyer used to have busses between IAD and DCA but that these were ended years ago.

You didn't mention BWI. Flights often cheaper going there. It's closer to Baltimore than to DC, but you can get to DC by MARC or Amtrak trains (shuttle bus from the terminal to the BWI train station and train from there to beautiful Union Station in downtown DC). MARC trains cost less but run only on weekdays and can get crowded with commuters. Amtrak trains cost more but also have more space for bags. Rating things just on convenience, I'd put BWI after DCA but ahead of IAD.