My co-founder was very experienced in php, so we said, lets find the middle and pick Python. Of course that's not the only reason. Python has great packages text processing (esp NLTK) and we really liked the simplicity
I mentioned RoR, because that was what I've used in my previous project.
RoR's learning curve is much more higher than Tornado. I don't even think that is fair to compare those two.
IMHO, RoR is more suitable for apps that requires lots of objects, views, etc.. E.g: A business app or a website.
In our case, we don't have that many tables, but lots of code around it and our app also runs on top of a single page. All the rest is taken care via AJAX.
One more factor in our decision was simplicity. You can easily dive into Tornado's code ( web.py ) and understand, whereas in RoR you need to read a lot of code, and Ruby is also making it harder to find out the flow of the code with its advanced concepts ( was called extends I believe )
And I am not a big fan of Active Record as well. We use PyMongo to communicate directly with MongoDb.
Tornado's Asynchronous abilities also an advantage.
Also I did not have that much of experience to use RoR in full scale, and if I got stuck on something, I need to read a lot and I don't like that.