If you're trying to get lots of jobs, PHP. Python and Ruby are both cool. Python is my language of choice by far. The main difference is probably in the philosophy behind the language design.
Matz created Ruby with his love of Perl at the forefront, so the language has a lot of flexibility and tries to make the programmer happy. This means the language design is a little 'messier', but the syntax can be more expressive.
Guido create Python based on his background making educational languages. He wanted Python to be easy to learn and use. There is only one optimal way to do things in Python (although you can approach the same problem from different paradigms, so there is a lot of flexibility, there is usually a 'Pythonic' way). This makes Python really easy to use and the design is 'cleaner'.
Either language is great. The Python interpreter is faster, and therefore cheaper to run on a server. You can use Django and get the same benefits you'd get from Rails. If you go with Ruby, it's probably worth checking out jRuby.