If you're a beginner, I would recommend you try Django if you decide to start learning Python. Of course if you decide Ruby is your choice of flavor, Rails is the obvious way to go. Whichever language you choose, I can assure you it will be a good choice.
Having said that, my personal choice is Python. I like the language, I like the community, and I use Python for almost every occasion. I use it for command-line apps, GUI apps, and I use it for web apps (Django). Oh and I use it for system administration scripts on Windows and Linux as well.
Having said that as well, I would recommend you learn a language like Haskell or Lisp as well. That will really open your eyes to a new perspective to programming. Furthermore, since you say you are mostly familiar with the .Net framework, I would really recommend you start with F# since you'll already be familiar with the libraries and it will make the transition much more smoother. Either way, good luck.
