Smart move.

If visiting DC, staying in DC is part of the experience. A hotel room is a hotel room is a hotel room. But, nice restaurants within walking distance, many of the museums, monuments and the like within walking / Metro / Uber distance, makes a limited trip much more fun and useful.

Car rentals at DCA are expensive (look at the extra taxes and fees). Car rental return at BWI is at a place labeled BWI but a 10+ minute shuttle bus ride involving crossing the interstate.

At worst, an Uber from DCA to downtown DC will run in the $20 range all in and an Uber out to BWI around $50 all in. MARC / Amtrak run out to BWI and DCA is 3-5 Metro stops from downtown depending on route.

Hotel parking, whether in & out or not will run $40-60 / night + taxes.

Day rentals often available for $25-30 for the Shenandoah run.