Really depends on what you want to advertise. In my own experience and the niche I work with, Twitter has the "brainy" audience, while FB has the "commoners". I tried app installs with Twitter, and got a ridiculously low ROI (for a game), but on FB I got around 50 installs. I tested another, more 'intellectual' ad on Twitter, and it performed slightly better. Also, Twitter sucks as their restrictions for ads are horrible. If they reject one to three of your ads, they are likely to ban you from advertising.