Numbers on Compete (http://siteanalytics.compete.com/foursquare.com+gowalla.com/) suggest that Foursquare is pushing quite a bit more traffic than Gowalla, at least on the web. Most reports suggest that Foursquare is also beating out Gowalla in terms of users.
(Full disclosure: I have friends at Foursquare, Gowalla, and on the Lift project. I don't really care about any one of them "winning", I just like facts.)