I have also stayed at both hotels, most recently for five days at the Park Hyatt. Although they are only a few blocks apart the Park Hyatt's location is far superior, right on the Avenida Alvear in the middle of Recoleta. The FS is right at the edge, next to a highway interchange and nowhere near as nice. As for service I though both were excellent but the Park Hyatt blew me away. And it's worth it to stay there just to have drinks or a meal on their exquisite terraces.