I have stayed at both and give the Hyatt a slight edge. It's less frumpy and more contemporary for one thing, but of course that's a matter of personal taste. Definitely a nicer location, especially if you like to walk places. The spa and gym are better too if you're going to take advantage of those. Here are two detailed reviews from Luxury Latin America:
Four Seasons Buenos Aires review
Park Hyatt Palacio Duhua review