Very interesting question! I would say Google Adwords simply because it's easy to set-up and doesn't cost as much as Facebook advertising. Also since Google ads incorporates quality score you can save a lot of money by simply making your landing page content relative to the keywords you are targeting with your ads. In the Adwords auction bids that win are not always those that have the highest big but rather those that have more relevant content.
Facebook is known to be really expensive and it takes an expert to set-up a campaign