Agree 100% with the assessment of the Marriott as old but yet regal. It’s worth staying at least once if just for the name and experience. After a couple of days the image wears off and the reality of the older hotel takes hold. The location can’t be beat.
However, I would argue for either the Sheraton (just a 3 minute walk down a hill) or for the Hilton. I have stayed at the Sheraton Towers several times and would recommend it. I did not have a problem with the rooms and the views from the upper floors add a little something.
If you have a chance, going over to Colonia (sp) in Uruguay is a nice way to spend the day. Its just a short and easy boat ride away.