I wouldn't count the need for a good JVM implementation as a plus mark in a Ruby vs Python comparison. Jython is used when you need to interact with Java libs, not because the C implementation of Python language is slower than the JVM one. The same does not apply for Ruby, you use JRuby because it's better than CRuby. If you want to compare speed you'd be up against pypy.
When a language has multiple viable implementations, it means the language has a good specification. It also means that it doesn't depend too much on platform-specific characteristics. It means it is more portable. Python is both blessed and cursed with a couple of problems ... its behavior is sometimes related to the CPython implementation (e.g. reference counting, __del__), and also some libraries are too big and important to live without them.
One such library is NumPy. Currently you cannot talk about an alternative Python implementation if you don't have NumPy running on it, and that's a fact.
I'm a Python developer in my day job and I never used Pypy for anything. I only played with it and became frustrated that libraries I relied upon don't work on it.
