Actually I did give a thought to Node.js/express and more pleasantly, the recent one called Mean.io. Mean.io is especially appealing to newcomers like me because it hides significant complexity in linking up stuff on the node.js platform. However, the only thing which puts me off is the instability/novelty of node and consequent unsuitability for production environments. Django OTOH has been battle hardened, and there is help available if I hit a wall. Not at all to say that the node community is not helpful. It's vibrant and helpful but there are simply too many gotchas yet to be resolved, and more importantly, discovered and I do not want to be the one doing these discoveries, midway through my startup. :-)

Regarding ROR, I'd say I have never even seen the language or the framework. Since I have done previous work in NLP/ML I was inclined to learn python so that I can utilize the huge spectrum of Python based ML/NLP libraries, which I believe Ruby lacks.