Foursquare always had a better design. Hear me out:
Design is about 2 things: how it looks and how it works. Gowalla always had a prettier coat of paint, pretty interactions. But what mattered more was the design of the game - becoming mayor on Foursquare was easier to grasp and get addicted to than dropping the original Gowalla teddy bears.
Coat of paint and nice UI doesn't always win.