From what I have been researching, there is only one crucial difference between the 2 languages.
JRuby imposes no type on variables. This makes it extremely difficult to use Java's library because you will have an headache with overloaded methods and classes with generic types.
Groovy has type as an option. You can choose to have a type or not. With this, you can easily use Java's library.
And another thing to mention is that if you dream about using JRuby on Rails, then you should stop now. Many gems are written in C, which is impossible to use them with JRuby.
So think carefully before using JRuby because you would have to reinvent many wheels.
