There are two questions you have to answer to decide the framework that is best for you.
1. Do you eventually want to learn to write GUI applications as well as games?
2. Do you want your games to run on Windows and Linux as well as Mac OS X?
Cocoa's main advantage is you can write GUI applications quickly. Cocoa would be a good choice to write a level editor for your game if your game needs one. If all you care about is writing games, there is not much advantage to using Cocoa.
If you want your games to run on Windows, Linux, and Mac OS X, I would recommend using SDL, which is a game library that does everything GLUT does and more. What is nice about SDL is it has supplemental cross-platform libraries for loading images (makes OpenGL texture loading easy), playing sound, and networking. SDL can also work with OpenGL if you want so you can use OpenGL for your graphics and SDL for things like reading input from the player.
The SDL website is located at the following URL:
http://www.libsdl.org
If you go to the Tutorials section, you will find a link to an article I wrote that shows you how to use SDL with OpenGL on Mac OS X.
Mac OS X (10.4)