PyObjc vs RubyCocoa for Mac development: Which is more mature?
Both are roughly equal, I'd say. Better in some places, worse in others. But I wouldn't recommend learning Cocoa with either. Like Chris said, Cocoa requires some understanding of Objective-C. I like Ruby better than Objective-C, but I still don't recommend using it to learn Cocoa. Once you have a solid foundation (no pun intended) in Cocoa/Objective-C, then the bridges can be useful to you.
I've been wanting  to have a play with either Ruby or Python while at the same time I've been wanting to do a bit of Cocoa programming.
So I thought the best way to achieve both these goals is to develop something using either a Ruby or Python to Objective-C bridge (PyObjc or RubyCocoa).
I know that ideally to get the best learning experience I would learn each techonology independently but I don't have the time. :)
So my question is which is a more mature platform, PyObc or RubyCocoa, main things I am looking for:

Documentation of API
Tutorials
Tools
Supportive Community
Completness of Cocoa API avaialble through the bridge 

Regarding point 5 I don't expect that the entire Cocoa API will be availble through either bridge but I need to have enough Cocoa APIs available to develop a functioning application.