If the mission and market make sense, then you bet on the leader.
It's not about checkins, it's about being driven to new great places that will delight you in the real world. Foursquare is the leader in that. Google:online traffic :: Foursquare:offline traffic.
Go ask any random person in a restaurant or spa where they heard about that business. What do you think most answers will be? Yelp or Foursquare?
Foursquare is not built as a search engine, first, while Yelp is. You check-in first via Foursquare, then you can explore other places, while in Yelp, you're prompted to search for a place.
If you're talking about simply discovering random places, then perhaps Foursquare is the winner, but that market is nowhere near as big as searching for a place. The latter is about capturing intent while the first is about generating interest.
It's not about checkins, it's about being driven to new great places that will delight you in the real world. Foursquare is the leader in that. Google:online traffic :: Foursquare:offline traffic.