You probably can't go wrong with either Django or Rails. From my personal point of view I'd favour Rails. In the past at least, Django also was more about creating CMS-type applications whereas Rails' main focus always has been SaaS-type business applications.

There's an argument to be made for isomorphic / universal code as well, i.e. using JavaScript both in the back-end and the front-end. However, if you're fine with learning two languages I'd say that the productivity gains by mature frameworks such as Rails or Django outweigh the reduced cognitive load of having to work with only one language on both the server and the client.