Having done projects in both languages, I don't think you will find a clear advantage in one or the other when it comes to power of expressiveness. As it was already said, it is a big matter of taste.
For instance, I like very much with Ruby that Arrays and Hashes can be dealt directly instead of going via references, and that classes are "first class values". At the moment, an advantage of Ruby (depending on your application) might be that there is also a Java implementation of Ruby available (JRuby), which makes it easy to interface to Java classes.
On the other hand, I don't like some aspect of scoping rules in Ruby.
For both languages, I found the community very friendly, so it is comfortable for a beginner.
For people not knowing either language, I have the impression that Perl is easy to lear, while Ruby is very easy to learn. I got this impression from talking to various people who actually started with one or the other languages, plus my own impression. If you find the time, implement some small task in both languages and decide which you like better.