Things I love about Ruby (I have not used Python yet):
- everything is an object, so you can e.g. store code in a Proc to execute it later or add new methods to Numbers (e.g. 1.kilobyte => 1024)
- very clean syntax and expressive method names means you don't need to document much, it's pretty obvious most of the time (e.g. 3.times do print "ho" * 3 end => "hohoho")
- very flexible, allows nice DSLs that read a lot like english (e.g. Rails or RSpec)
- interesting community with lots of free material (e.g. _why's poignant guide, railscasts.com, therubyshow.com, ...)
I'm sure Python is pretty cool as well, but from all I've heard (I had the same question a while ago) Ruby seems to fit me better with its "more fun and freedom" philosophy.