21 P or Mark & Orlando's ?
There are lots of good restaurants in the Dupont Circle area. While 21 P and M and O are close, they can also walk to Firefly, Hank's Oyster Bar, Komi, Nora, or The Tabard Inn. Of the lot, I'd recommend Komi or Nora for expensive and needing a reservation, or Firefly or 21 P for a little less and easier to get into.
Forget 21P. It's now closed. I think it closed early last week. Looks like they didn't file their necessary permits....
I've eaten at Mark & Orlando's 3 times in the past 3 weeks. Excellent every time. The grilled caesar salad is so good. Ask the waiters what is freshest. They won't hesitate to suggest and their recommendations have been spot on each time.