This is a loaded question – BOTH!
Usually Google because everything has an 80/20 rule – and this is no exception. Google rakes in over 83% of all researches on reviews, while Yelp only brings in 13%. However, Yelp has been seen as a higher conversion rate for searchers and can actually play a role in whether your business will THRIVE or DIE (if you don’t have Google reviews). We have also found that when a client has a great Google Reputation and a so-so / bad Yelp rating – things tend to balance out and the business will still grow without being hurt too much by Yelp.