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.
What? No mention of IronPython?
IronPython is the flagship language of the DLR. It allows you to use all the familiar .NET libraries, but through Python.
I would definitely try Python and IronPython. You'll learn a lot and might even sneak it into your current projects (you can embed an IronPython engine in a .NET application).
Python or Ruby for a .NET developer?