Hey gtalwar! The Palacio Duhau Park Hyatt (PH) is one of the best hotels in Buenos Aires. Although the Sheraton is really good, the PH will give you a better experience IMO. The location is a lot nicer, specially to go out in the evening for a walk/dinner and so are the services and rooms. I think its worth the cash "upgrade".
Let us know if you need recommendations on what to do, where to eat and what to drink .