Are you going to write your server side code from scratch or use a framework? If you're going to use a framework, it would be smart to pick your language based on the framework that is the most inspiring one for you.

Both Ruby and Python are fine languages. You can't really go wrong with either one. To me personally, the languages have a different vibe/feel. Python is easy to read. It sort of looks like pseudocode. Ruby, on the other hand, usually feels a bit more cute and clever. Once you get the hang of it, it's quick and fun to get things done with it. But that's just my opinion.