Python or Ruby for a .NET developer?
First... good for you for wanting to broaden your knowledge! Second, you are comparing a language (Python) with a web framework (Ruby on Rails).
I think your best option is to try a few different frameworks in both Python and Ruby, do the same fairly simple task in each, and only then pick which one you'd like to learn more about. Rails is nice for Ruby, but it's not the only one out there. For Python I like Pylons and Django.
Pros and cons: Ruby is a little cleaner, language-wise, than Python. Python has a much larger set of modules.
Is it worth learning? Yes, to both Python and Ruby.
I'm a C# .NET developer and I work on mostly ASP.NET projects.
I want to learn a new programming language,

to improve my programming skills by experiencing a new language, 
to see something different then microsoft environment,
and maybe to think in a different way.

I focus on two languages for my goal. Python and Ruby.

Which one do you offer for me ? 
Pros and cons of them on each other?
Is it worth learning them ?

EDIT : Sorry I editted my post but not inform here, 
Ruby on Rails replaced with Ruby.