I've had a half-dozen or so "luxury" bookings at DCA since July. I've pretty much always gone with a sedan (BMW 3, 5, and occasional 7; Audi A5; XTS, etc.). But I think they typically have at least a handful of what m907 described. One thing's for sure, when I say "I booked a luxury" they've always been really nice about letting me have whatever I want, and I took an SUV when I needed it (XT5?).

L'Enfant Plaza is indeed open and will have the car you booked ready for you (I used a free day there to get a one-way Tahoe to drop off in Vermont) but likely not much if any selection among different vehicles. The Tahoe I got was high-mileage but I needed the space.

I don't think BWI is much if any more convenient than DCA other than saving the extra 30 min on the train from BWI to WAS... at BWI, you have to shuttle from Amtrak to airport and back to Rental Center. Easier just to take an uber from Amtrak to the Rental Center instead of two long shuttle rides. Metro from Union Station to DCA also requires a line change.

Honestly I think the biggest factor is what day you plan to pick up. If you're picking up on a holiday-weekend Sunday or something when everyone has weekend cars checked out, there won't be much to choose from.