Ruby/Rails seems easier to develop and test the full stack application, from client JavaScript to server Ruby response and back. It supports Ajax through injection of injection of JavaScript into HTTP responses served up, based on specifications in server side code. The test architectures seem to reach across the client/server divide, for example checking server side responses to triggering client-side JavaScript.
Python/Django are trying, with like lettuce and splinter, but these don't yet seem to be quite to the same standard.
Also, because RoR has a lead on web development, new developments in that space seem generally better integrated. For example, Backbone is a young JavaScript MVC style framework for client side development. I've just been fooling with using it with Django, and discovered that its stringification of Json objects for POST doesn't work with Django's parsing of the client request. Maybe more experienced developers spotted and fixed this easily -- my fix is only a couple of lines -- but it's the sort of thing that makes me wonder just how many folks have tried to integrate Django w/ Backbone. I'm guessing that the Backbone community has already spotted and fixed any such issues with RoR.
Now I'm not sure how I feel about RoR, it often feels like magic and I'm a little suspicious of stuff like super-convenient JavaScript injection. Maybe when I understand it better it will seems less magical and more convenient. But it isn't hard to see why the web development community likes it.