The Sheraton was not completed yet when I was in Mendoza, but I would say that strictly on the basis of an ideal location, I'd pick the Hyatt. I didn't stay there either, though - I stayed in a good place for 1/3th the price that was 100 meters around the corner from the Hyatt called the Argentino. Somewhat small rooms, but a perfectly nice 3.5* hotel with a very helpful and friendly staff. And conveniently, it's next door to the Hertz office in downtown Mendoza, if you want to rent a car for wine touring.
http://www.argentino-hotel.com/site.html