Google AdWords have better potential to drive immediate, direct sales due to the nature of search. A customer may be ready to buy at the moment they search for a type of product and if your ad is appropriately placed, you can get the sell. However, the CPC bidding model of Google can be highly competitive and therefore expensive for your ad to reach top billing.
Google also has additional offerings, one being content marketing. Here your ad (text or graphic) is placed on affiliate sites with relevant content to your ad. Again, cost may be an issue as graphical ads are even more expensive than text. Apparently Google can provide television advertising with AdWords through Dish and DirecTV as well.
Facebook ads will less likely result in immediately sales but will build brand awareness quickly and fairly inexpensively. You can create multiple ads aimed at incredibly specific target markets with the most effect verbiage for that group. Pricing is also CPC or CPM bidding but much less complex than Google's. The interface to create an ad is very intuitive as well.
In my research, I have found that search marketing is VERY effective .... unfortunately, it is also mostly too complex for the uninitiated.
Adwords is the dominating search marketing platform, but Google's rules change so frequently, and their guidelines can be so confusing.
Furthermore, putting together an ad with proper keywords requires extensive keyword research. Most people scratch their heads at the notion of SEO.
This is what you need to know before you even create the ad. Then you need good ad copy, which is usually outsourced, since not everyone is a copywriter (this only adds to the cost).
Facebook PPC on the other hand, is quite simple.
It is totally user friendly from start to finish. It also gives you a lot of options when it comes to narrowing down your target demographic. It is also incredibly inexpensive, whether you use a ppc or cpi campaign.
In the end, I do believe that if you are selling something, the fastest and most effective way to do so on the internet is through search marketing, which would be Google Adwords...but, unless you are willing to put forth the immense effort to learn Adwords, or you have deep enough pockets to simply hires an SEO/PPC/Copy expert to do it all for you, your best bet is to take the somewhat slower, but still highly effective approach of Facebook PPC.
This strategy is more about branding. Branding, however, is also a very useful way to build sales.