I fly to DTW/CLE a lot from DCA and when we take off to the South, most of the time the plane banks to the West after clearing Old Town Alexandria and the view of greater DC can be pretty good on the right side (especially on clear days) even though you won't see Mt. Vernon. If you take off to the North, I actually like the Pentagon/Arlington side better because you can still see a lot when making the turn to follow the river.

Regardless, DCA, IMO, is one of the more interesting airports in the country to fly in and out of so enjoy whichever way you take off!