Not quite sure about what you mean with "more professional to use"? It's not the tools that defines whether something is professional or not...
Cocos2D is an Objective-C framework adding a level of abstraction to OpenGL ES, making it easier to handle. People who already know OpenGL might prefer working directly with it at a lower level and some might already have created their own frameworks.
Cocos2D seems very robust and efficient, so whether there is anything to be gained from going at it yourself is an open question. Personally I love Cocos2D and simply don't have the time (or brainpower?) to learn something as low-level as OpenGL when there's still so much Cocoa-stuff to learn. If you're just starting out I would strongly advice against trying to learn OpenGL at this stage.
