I would say it depends. Facebook does well with home, artistic, and visually appealing items such as food. Facebook is also a great place to advertise things that may not have a great search volume on Google.

Adwords, on the other hand, is more costly but you are going to get a higher conversion ratio if you set your to account up right because somebody googles a service or product is likely looking to buy. On Facebook, they get shown the ad if they "may" have an interest.

That's why if you are selling services or products that are big-ticket items I would say use AdWords and maybe also facebook. Don't try sell small things on Google Adwords directly, rather get in with their store.