imho the answer must be given in relation to YOUR project - I'm currently in favor of Groovy, as keeps much closer to the core Java platform. JRuby, on the other side, has a rather large Ruby-community as backbone...
A small writeup of the topic, including some interesting comments, comes from Kimchy.
For you guys loving photos, an interesting stack:
They forgot, what a shame, the fantastic Groovy-in-Action, one of the best programming books I devoured within the last year(s).