Martin --

I think you are probably best where you are as a first time visitor. The only one I have stayed at recently is the Ren Dupont Circle which is not really at Dupont Circle, the RI Dupont Circle is actually closer.

The Ren Dupont is in a very nice semi residential portion of the city but is not near any of the sites save the Phillips Collection, however the metro is nearby. The rooms are modern but small.

I think it would be helpful to explain how many days you will be there, whether you will be there one weekdays or weekends and whether you have status in Marriott.

If you will be there weekdays, then the RI may be a good choice, as well, as you will get evening snacks.

Please elaborate more and I will try to help.

Also, the moderator of this forum, Bruce, I believe is a big booster of the metro center.

Read the reviews of the indiviudal hotels you are considering here and on tripadvisor to give you an idea of the type of rooms offered by each hotel and how recent their renovations were.
PS -- I hope you enjoy our nation's capital, I really loved Oslo when I visited your capital!