IMHO, the language is the stronger point in all of these. I don't care if the JVM is well specified, this brings me nothing to my daily productivity.

What is the advantage of using Rhino on JVM against using native Ruby or Python? In Java, at least I know I will be very confortable doing automatic refactorings with Eclipse, which is the biggest advantage of Java against these other languages. Having coded a tool in Ruby with some good testing coverage, I still don't feel as safe to refactor as I was using Java.