I'd choose groovy. It's a JSR, the syntax is Java like, it feels like part of Java, etc. It doesn't feel like a whole new language to me. Instead it feels like an extension or part of Java. For example, when writing web apps in Struts 2 or JSF (via Seam, or JSF 2.0) it is possible to use Groovy to write the code behind the web app. This can make web development faster.
Ruby on the other hand is a whole separate language with a syntax that keeps me from even considering it. The rudeness of the Ruby community is also terrible and I do not want to support their Java bashing cause.
I love Java. Why should I even care about Ruby when I can continue to use the language and frameworks I love, backed with a Java like JSR scripting language.