Currently there's quite a debate raging over the relative merits of Groovy and JRuby as scripting languages running on the Java virtual machine. Curious minds want to know - which of these languages will win this upcoming language war? People want to know which language to pick for a project, or which language to commit to learn. Perhaps the first thing to point out is that it's perhaps rather unfair to see this as a race between these particular two horses. Scripting has been available on the JVM for a long time. Jython, a Java implementation of Python, has been around for several years. There's plenty of other, more obscure languages, which I daren't mention for fear of offending all the ones I miss out. It's too early, or rather too difficult, to say if one language will win out. Both are pretty young, only just finding their feet on the JVM. On a more personal level, your choice has a lot to do with what you expect to do with it. If you are only interested in running on the JVM, then Groovy could well be the easier choice. You are working directly with Java's library and object model, and the syntax requires less getting used to. A strong reason to prefer Ruby is the fact that it lives in multiple implementations. Ruby is a tool you can use in a lot of other places. As a long time Rubyist, there's not much incentive for me personally to get heavily into Groovy, even though I actually like the language a lot from what I've seen of it.
Read Martin Fowler's complete post: http://www.martinfowler.com/bliki/GroovyOrJRuby.html