Gowalla, you have the check-in aspect that lets your friends know where you are. You have incentive to go to different places and explore new venues because you get more stamps that way. You have the chance of getting a bonus item at any new venue you check in to or you can trade in items to add to your collection when you go out. You also have the trips and challenges that make it fun and exciting when you reach a town you've never been to. I checked into every ride I went on at Disneyland just to get the next beautiful stamp and to get the Disney challenges.
Unfortunately, foursquare is the media darling, the first, the one with all the hype. It shouldn't be that way. Gowalla app is more stable, design is infinitely superior and the game play is a lot more exciting and engaging. Foursquare just has good PR and the ability to facebook it up by making all kinds of deals with different companies. Bor-ing.