Well, to me Ruby as a language seems nicer and more powerful than Groovy. Having said that, Groovy can be easier for a Java developer to learn since a lot of its constructs are almost like valid Java code.
Both Ruby (through JRuby) and Groovy can be run on a JVM and thus integrate with Java libraries so there's not that much of a difference, although integrating Groovy to a Java application is probably slightly easier.
I feel Ruby is a much more powerful language than Groovy. Groovy has a lot of language features which seem to be similar to Groovy and Python, and it has great integration with JAVA itself. But writing things in Groovy just doesnt feel very natural to me. With Jruby now you can have the same access to the JVM, while harnessing the full power and expressiveness of Ruby.