The Renaissance hotel is really far out. Very little to do there without a car. City Center is located in the to the quieter end of downtown and you could walk down to LoDo, but frankly I think the Courtyard is the best located hotel in downtown Denver. Close to LoDo and the strip of bars and restaurants along Market.
Of course, either the Marriott or the Courtyard probably charges a fair bit for parking, so that might play into your plans.
Edited to add: Oh, and the Courtyard is a very nice hotel. No lounge, but the rooms are quite big and well appointed.
Last edited by GregWTravels; Aug 7, 2008 at 10:55 am
Reason: Clarification on which hotels charge for parking and adding nice words about Courtyard