cocos is faster and more flexible, and since its open source it can be optimized for your games specific needs.
logic in unity is handled with scripts, which has benefits and drawbacks. unity also has a lot more features out of the box, especially when it comes to audio (which will be changing with 3.0 or 3.1)
I think it really depends on the kind of game you want to make. If you want to rapidly make a cookie-cutter game, I think it would be faster in unity. I have considered moving to unity more than once, but have ultimately decided to stick with cocos for 3 main reasons:
1) im building a shmup, so performance is crucial. I like being able to have full control over whats going on, instead of relying on unity’s engine.
2) im using c++ libraries, which are possible to use in unity, but require you to build plugins and buy unityPro (expensive)
3) Levels in my shmup will be procedurally generated, which makes the scene editor in unity useless for me. Some games might make great use of the visual scene editor, but not mine.
I suggest finding a finished game in unity with similar requirements to your own, and asking the developers what their experience with unity was. And then do the same for a game in cocos