How do you know it is the same plane? Just because both legs have the same flight number doesn't mean that there isn't an aircraft change at DCA. Some direct flights have scheduled aircraft change while all other direct flights are subject to last-minute aircraft swaps.

Regardless, it is generally best to de-plane in order to get on the VDB list - or possibly score an upgrade (if you are into that); especially if your inbound leg arrives at DCA early.