Jython or JRuby?
In both cases, most of the code should Just Work™. I don't know of a really compelling reason to choose Jython over JRuby or vice versa if you'll be learning either from scratch. Python places a heavy emphasis on readability and not using "magic", but Ruby tends to give you a little more rope to do fancy things, e.g., define your own DSL. The main difference is in the community, and largely revolves around the different focus mentioned above.
It's a high level conceptual question. I have two separate code bases that serve the same purpose, one built in Python and the other in Ruby. I need to develop something that will run on JVM. So I have two choices: convert the Python code to Jython or convert the Ruby to JRuby. Since I don't know any of them, I was wondering if anyone can give me some guidance. Like which one runs faster, or more importantly which one has tools available for easy code migration(.pyc to .jar files)?