I would buy traffic from where the buying intent is high. For example, people searching on Google Shopping are actively searching for products, where Facebook the buying intent is less.
However, if you use a combination of Google Shopping and Google Display Dynamic Remarketing and Facebook remarketing, then you are getting the most in both worlds. Then you can go even further and use Facebook lookalike ads.
For starters, I would recommend you research where people are buying actively than advertise there.