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:
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.
I would echo Chris' assesment and will expand a bit on why you should learn Objective-C to learn Cocoa. As Chris says, Objective-C is the foundation and native language of Cocoa and many of its paradigms are inextricably linked with that lineage. In particular, selectors and dynamic message resolution and ability to modify classes at run time are required to implement Cocoa technologies such as Distributed Objects and bindings. Although these features are available in other dynamic languages such as Ruby and Python, there is enough of a mismatch in the language models that you will have to at least understand Objective-C to understand Cocoa. I suggest you take a look at this previous question for further discussion: Do I have to learn Objective-C for professional Mac Development?
Fortunately, Objective-C is very easy to learn. I often tell people it will take them a day to learn Objective-C coming from C/C++/Java or LISP, Scheme, or any of the 'newer' dynamic languages such as Ruby and Python. In addition to expanding your mind a bit, you'll learn to at least read the code that is used in virtually all of the Cocoa documentation and examples.
As for Ruby vs. Python, the bridge capabilities are very similar. In fact, they both use Apple's BridgeSupport (shipped with Leopard) to provide the bridge description. Both are supported by Apple and ship with Leopard. It's a matter of personal taste which language you prefer. If you choose Ruby, I suggest you give MacRuby a look. It's definitely the future of Ruby on OS X, as it reimplements the Ruby runtime on top of the Objective-C run time. This provides some nice performance and conceptual advantages (including integration with the Objective-C garbage collection system, a feature currently lacking in PyObjC which uses the native python gc). MacRuby also includes a custom parser that makes the syntax of bridged objective-c methods a little nicer. The downside of MacRuby is that it's not quite ready for production-level use at the time of this writing (June 2009). Since it sounds like this is a learning project for you, that's probably not an issue.