As bairaghi said, depends on kind of business, moreover type of customers, are they from 15-45 year range, do they use cell phone etc, if your business is service sector, yelp will be fruitful, I am in service sector and yelp has been best for me
Yelp is free to use,if you provide best service and if your clients gave you good reviews, then there is no cost, advertising in yelp costs money and I have not used it and its not worthy if you have good service and 5 star rating , good luck