There really isn't a huge difference between the two. For what it's worth, I personally prefer Ruby because it has better support on the Mac - Apple use ruby a fair bit themselves. For example, the command line tool gen_bridge_metadata (supplied by Appleto make it possible to access system frameworks written in C from scripting languages) is itself a ruby script. I notice that on Linux it's the reverse - python seems to be the preferred scripting language.

But don't take my comments as saying only ruby is supported on the Mac, or only python on Linux - in reality it's only a light preference one way or the other.