Maybe I misunderstood your question. If you're looking for reasons to pick one over another, then it's whichever language you know best.
Web frameworks really only differ in philosophy, and in this case, both are batteries-included, monolithic frameworks. They are practically identical solutions.
Philosophically they're quite different. Every layer follows a different approach: templates, controllers (views in Django), urls, model definition, ORM etc. A lot of this comes from the core philosophies of the languages, whilst Ruby and Python may be closer to each other in philosophy than either are to say Java, they still have substantial differences.
