I know neither Python nor Ruby at this point in my life, nor do I know PHP. I'm a pretty strong Perl programmer for general processing, although I've never built anything complicated, like a web application, in Perl. I have decided to learn Ruby, for two reasons:
I don't really look for "power" in a language. I look for code readability, a simple and stable core, and portability. I have to admit that I prefer "strong typing" and languages with compilers that prevent you from making lots of mistakes -- in that respect, I think Ada is king of the hill. I like the discipline to be built into the language and not supplied externally with processes and CASE tools. But I also realize that most programmers I need to work with won't touch a language like that with a ten-foot pole. The "new" languages -- Perl, Python, Ruby, PHP, etc. -- seem to be where the action is.