--1.9 mode works really well under JRuby now - that was a big push within 1.6 and they've made a lot of progress. I'd call it "almost there".
Rubinius is a little bit further behind, but I've been able to run our Goliath stack on it, and that exercises quiet a few syntactic changes. So, I think both are close.
Having to choose between multiple threads and a better language is really not a decision you should have to make.