HTML5 is becoming so powerful that it is very much possible to develop games for mobile platforms using them. If you are planning for simple 2D games, then I would say you go for HTML5.
But if you want to do some big complex games supporting device orientation and other such features, then I suggest you learn Objective-C. Because as of now, there in no direct procedure to make the game stick to one orientation (which I suppose is one of the basic feature). As far as I searched it is not possible to tell the browser to stop auto-rotating on change of orientation.
Another things is about performance. The performance of canvas API inside the browser is good enough for simple games, but it cant compete with the power of native games.
Regarding libraries, on native side there is cocos2d and many other mature libraries; but on HTML5 side I dont think the libraries are not mature enough when compared to native libraries. But soon this problem will vanish as the libraries evolve.