I just want to know, that in the long run, using which of these engines will be better. Although I feel that using Cocos2D will be a better option, as it can also be used for iphone development, there's just 1 tutorial http://dan.clarke.name/2011/04/how-to-make-a-simple-android-game-with-cocos2d/ which I've found so far. libgdx, on the other hand has a complete book.
Consider using AndEngine. From a short look at libgdx, they seem similiar (Although AndEngine is 2D only). However, AndEngine has everything you need for a 2D game. The only thing it lacks is documentation, but it is open source - if you don't get something, take a look at the code.
There is an active forum here, and there is also a SO tag for AndEngine, so you can ask questions here too.
It also has many extensions for useful add-ons, like the physics engine Box2D which is a 2D cross platform physics engine used for many well known games (For example, Angry Birds uses it). It also supports TMX tiled maps, with a pretty easy interface (Actually that's the game I'm developing now, it's really easy to use the AndEngine TMX utillities).
So.. before you pick one of cocos2d or libgdx, take a look at AndEngine :)
@Greg That's not true...
It is easy to learn and good documented engine
There is almost no javadoc at all. But the forums are full with tutorials, and so is the web.
EDIT: From your comment at Greg's answer, if you are looking for easy developing, AndEngine is a good choice. From taking a look at that's cocos2d tutorial code, it seems much more complicated and less intuitive than AndEngine's code.