The Park Hyatt is a far superior hotel in my opinion. The Sheraton was built as a convention and group travel hotel and is much larger. The Park Hyatt is more intimate and (in my three nights there) has an excellent staff. Here's a detailed review of it:
Park Hyatt Mendoza.
As others have said, the location is far superior as well, especially if you like to stroll around a new city. And it's close to a park that's bigger than Central Park in NYC. You can walk to lots of other restaurants nearby.
Book a wine tour with a company there like Grapevine. Many of the wineries are by appointment only.