I would seriously consider Hyatt right now. They have a fantastic promotion going on right now where you can earn free nights after every two stays. Much like Marriott, they have pretty good coverage in the DC area, so you can switch hotels to maximize your freebies. Also, they have an "instant" Platinum promo - with a fast track to Diamond (equivalent to Marriott Platinum) opportunity, as well.

Hyatt has added/re-branded a lot of properties in the last few years, giving them much wider coverage than they had previously. I have been a Marriott Platinum for five years, but Hyatt is definitely going to steal some business from them this year.