Hello every one, I need your suggestion on 3D game development, I am an ios application developer, have worked on Cocos2d game engine. I want to further dig in to game development field. Shall I start to learn openGL or some game engine?

I would say, based on the fact that you have used cocos2d before, that you should look into a game engine. I seem to remember seeing a cocos3d project before which might make sense. I don't have any experience with 3d game engines on iOS though.

I personally have way more fun working with OpenGL directly. But if you're looking for a 3D engine that will get you up and running quickly, Unity is what all the cool kids are using.

yes definitely you can start to learn openGL because OpenGL is that the most widely adopted 2d and 3D graphics API within the trade, bringing scores of applications to a large kind of pc platforms.

You can also turn to
http://www.unrealengine.com/udk/
or
http://mycryengine.com/


Knowing OpenGL is always a great assett.