If you are really concerned with saving money (as you note) then WN is unquestionably the best choice for your needs.

If you are more concerned about "status" perks as you intimate, then WN is not the answer. However, if you do fly 40+ roundtrips a year you will easily qualify for a WN companion pass, which will save you or your company a lot of money.

Unfortunately WN doesn't fly from DCA, so if airport selection is part of your formula (and I could understand how that would be, given DCA's convenience and top-notch terminal facilities) then you will have to go with another choice. Note, however, that UA's schedule from DCA is pretty bare bones, so you'll probably end back up at US.