Id say go with the language you prefer. If you fancy yourself a Pythonista then go with Django. If you prefer Ruby syntax then go with Rails. Both languages are different to C#, or VB.NET which is where you are probably coming from so dip into each and follow the path of least resistance. Honestly when it comes down to productivity there isn't much to be gained from using one over the other. I guess one thing that is…
Another thing to consider is what your goals are here. Is this just learning for learning's sake, or is there a project you have in mind that you want to build/prototype? In my experience the differences between Django & RoR are minor, but your end goals obviously will also help guide you toward one framework over the other.