There are certainly slight differences between demographics on both. However, the best way to run your ad campaign is initially on both (as long as it works with your campaign objectives eg you can only run a page likes campaign specifically on Facebook) and then let the algorithms work their magic by showing it on the platform that is getting the best results depending on your objectives. It will split the budget out between the platforms and push more dollars to the platform that is garnering the most cost-efficient results for you...again depending on your campaign tactic and objectives chosen.

Ask yourself where does your target customer hangout?
In most cases we run ads on both.