[hcayless]

My impression is that JRuby is a lot more advanced, particularly from the point of view of parity with its "parent". That's not firsthand knowledge though: I use JRuby a lot, but Jython not for years, so I'm not plugged in to the latter. My impression is that its development kind of stalled out a few years ago, but maybe has recovered in the last few years.

JRuby performs very well, especially if you're using it in an environment like a web app, where JVM startup time isn't an issue. It's definitely better once you reach a certain threshold of usage. I wouldn't use it probably for little stuff, but if you're getting to the point where you need things like multithreading and garbage collection becomes an issue, then JRuby is a good bet.