Depends on your circumstances in part.
Will you be doing scientific programming? Python.
Graphical games? Python.
Web apps? Ruby or Python are about comparable.
Do you really really really want multi-line lambdas? Ruby.
Do you really want to make DSL's? Ruby.
Do you really want code that's easy to read and maintain years from now after it's no longer fresh in your mind? Python
Do you want a language with a smaller number of rules for your brain/tools to parse (ie. a more "regular" syntax)? Python.
Do you want as much "magic" and implicit behavior as possible in the language? Ruby
Do you prefer things to be explicit, but still concise? Python
Do you want to be seen as cool and hip as possible? Definitely Ruby rather than Python
Will you likely be writing lots of CPU-bound code and therefore need a language that has a fast runtime characteristics? Python
Do you think Perl is a good thing? Ruby. A bad thing? Python.
Do you want a lot of open source projects and a rich ecosystem? Ruby and Python are about comparable
Do you want to hob-nob with folks who are, on average, generally younger? Ruby
Do you think "end end end end end end" everywhere looks dumb as hell? do Python
Do you think having to type "self self self self self" a lot in classes feel wrong to you? do Ruby.
Want the one of the two that's most popular/blessed/supported by Google? Python