I had an awesome experience at that hotel in October and posted under the Hotels forum a thread titled "Unbelievable". The link below should get you there.
Try staying here Renaissance Washington, DC Downtown Hotel.
I got a 109 a night rate with parking. The hotel is two blocks away from the White House and 4 from everything else.
I've stayed at the Marriott Wardman Park at least twice for conferences. It was fine. Last time attending a conference, i opted to stay at the Renaissance nearby and it was fantastic. It's an historic property and in a great location. My room was small but well appointed. Parking fees are high, as is the norm in a large city. Try walking over to Firefly (restaurant) - it was great!
I've not stayed at either property noted but others in the area, including the JW and Courtyard. Both of those properties were great and even with points the JW offers some amazing rooms/views. If you are focusing mostly on the museums, etc around the national mall, you can walk most places and take a cab/train from the airport. Unless you are leaving the central downtown area, a car is more trouble than its worth. Should be a good time to be down in DC though, a bit early for the cherry blossoms but they will at least be starting to bloom in mid/late april :)
You have some great suggestions!
Hope your trip is after the 15th!
Having stayed at most of the choices, my favorite has always been JW and then the Ren. BUT, prices very so much, you will need to shop, including the computer, and reservations, finally do not forget to call your choices directly!
The JW, puts you in walking of most everything, but the "Trolley" is a great way to see it all. One thing to enjoy on a good weather day is Arlington! Just remember there is no food there, so do not go hungry!
Have a great time!