Ruby is slightly newer and wasn't particularly popular in the United States until it was popularized by Rails.
Python became popular with Linux and open source developers before Ruby did, and has had slightly more time to develop a nice set of libraries to do just about anything.
May I also add that Python works very nicely on a Windows box. Ruby/Rails works on Windows, but when I used it, it hobbled around on one leg. Going to PyCon this past year, I was stunned at how many Windows laptops there were. Python itself and the Python community are really quite OS agnostic.
The online Python documentation tends to be pretty good. When I picked up Ruby a year and a half ago, and I tried to find the documentation, people generally pointed to the pickaxe book as the de facto documentation.
Yes, the pickaxe book or the class docs at ruby-doc.org
Although I've been using Ruby on windows the last few weeks with Ruby Shoes from _why(normally use for unix scripts or web-dev). It's pretty nice (although a little rough around the edges as it's under heavy developement).
When you install shoes (the latest version anyway) you get the GUI tools, ruby gems, and sqlite, and a ruby interpreter, all neatly packaged. And there is a whole load of items I havent looked at yet. It's fun.
It's really easy to develop portable Python applications that run equally well on *NIX, Windows and OS/X. I mount a single share on Linux, Windows XP and OS.X systems and run the same Python program and have them run identically on all three systems, side-by-side. I'm not saying that you can't match that with Ruby, but for me it's a way of releasing programs concurrently for all major platforms.
Come to think of it, we, ruby programmers, don't really pay our dues. 1.9.1 has been out since January/February, and nobody bothered to package it for OSX, Windows, or Ubuntu.
Python became popular with Linux and open source developers before Ruby did, and has had slightly more time to develop a nice set of libraries to do just about anything.