Hmm, I'm gonna be honest. I received the contract for joining Fullscreen, and based on its terms, you'll be giving up A LOT of control to Fullscreen, especially if your channel is very popular. I'm not really sure the benefits of joining them are enough to outweigh the enormous loss of controlling your own content as well as paying them part of your profits. People have said that they "believe" joining fullscreen will give them a higher share of revenue than through Adsense, but I'm really not seeing any strong evidence to support this, especially since it isn't clear exactly how much YouTube takes through their partnership program.
Also, whoever said that you can leave Fullscreen at any time is wrong. When you sign with them, you are locked in for at least two years. That's a pretty long period if you ask me.