I tried both and I found out (for my particular niche) that on average the facebook ads were more expensive and not as effective. But I hear some niches work really well on facebook (such as dating, gaming, etc). So I guess it all depends on the niche.
I was doing CPC. I had to pay about $0.63 on average for my classifieds ad. AdWords usually ended up being $0.25-0.40 per click.
I have also tried both and Facebook ads work better for me. I mean profit is more in FB when compared to Adwords.