This.

For most of my hobby projects, learning a new language is the main goal.

It depends - Rails has so many conventions used (magic behavior) that, while useful, you have to learn them all and even how to override that behavior, before being able to tackle anything else other than simple blogging software; although the learning experience is enjoyable.

On the other hand Django is an (arguably) thin layer on top of Python. You can start from the URLs specifications and how to construct a simple HttpResponse object. And that's all you need to know and how tutorials teach it -- for everything else you can rely on your previously learned skills, while learning shortcuts on the way.

For another perspective -- Rails learning is top-down, while Django is bottom-up.

Yeah, I definitely learned way more about Ruby when I used it to automate a set of tasks (like my build/push process) and writing something minor like a twitter bot or a text parser.

Using Rails on a larger project is good as far as learning a framework, but to get into the language and get into the cool/weirder stuff I've found small projects do the trick.