I have used facebook ads a few times and basically broke even. With Adwords I was in the red so I stopped using them. Which of these two have you had more success with?
Of those 2 I had the opposite. AdWords worked a lot better than FB, but was still not much better than break even. I have had more success with Bing ads, though it didn't bring in the numbers. Strictly from a return on investment though, Bing made the most profit vs. investment.
It really depends tremendously on your product, niche, geos, and many many other factors. Facebook is pretty much the gold standard these days for mobile app installs. Adwords obviously is still the king of intent, and results will vary for each. I will say though that Facebook has phenomenal targeting based off of registration data so you can really get granular with the interest/demos/behaviors/geos of an audience, while adwords is pretty much just by keyword/demo/geo.