Depends on your goal. Google Adwords is primarily "demand fulfillment" - because users tell you what they are looking for through keywords on search engine.
Facebook Ads is "demand generation" wherein you target ads to people on the basis of their interests, the pages they like, etc.
Now, where should you spend your budget? I've found that niches where adwords cpcs are pretty high eg. finance etc, it is better to go for facebook because you're not bidding on keywords but to interests. So you can get more prospects cheaper on facebook that way.
But in some niches the cpc are so low that you're better off catering to demand fulfillment through google adwords than waste your money on facebook.
Overall, google adwords gives a better conversion - because it is direct marketing. Facebook marketing is slight indirect in nature but you may end up reaching a wider audience that translates into building your brand.
I hope that answered your question.
- Anant Mendiratta