Hi César,
If you're talking about just doing some basic scripting, I don't think there is much difference across Python, Perl, Ruby, and PHP. I don't have extensive experience w. Python or PHP, but coming from a Perl background, I can tell you most of the sharp tools found in Perl can also be found in Ruby. As it turns out, Perl is faster than Ruby, so for the most basic things, Perl might be a better choice if speed matters.
The main difference I see is when a project grows beyond small one-off scripts into a complete application or library. When this happens, Ruby begins to shine (along w. perhaps Python) as a general purpose language that can pretty easily meet the needs of medium to large scale projects. Between Python and Ruby, the primary difference is in the object system. While Python's object system is sort of layered on top of the core scripting language in a sense, Ruby's is baked in, with the language being deeply object orient (in the Smalltalk sense) from the ground up. While Python may be a bit more mature than Ruby in some ways, most will agree that Ruby's object system is far more elegant.
Of these languages, while I think all have the ability to pass around anonymous functions, only Ruby provides code blocks as first class citizens. To see an example of these in action, check out this blog post:
Code Blocks - Ruby's Swiss Army Knife
So the main advantage of Ruby over PHP or Perl 5 is that it can handle complex object models without doing a lot of work to keep things clean. The advantages over Python are far more subjective, and come at a cost, so I think that choice largely boils down to personal preference. But of the 4, I personally think Ruby spans the greatest distance in potential purposes, from the most basic scripts to deeply complicated business applications. So if you had to learn only one, Ruby might be a good choice. But learning a little bit of each of them might be the best way to see for yourself what makes you happy.