Look it is just a web app framework. If you know Rails well, you should be able to pick up Django fast (and vice versa). Sure you'll get a few surprises in some edge cases, but hey, in the end it is just a framework that mediates between http requests to databases and generates web pages. How difficult would it be to grok either once you know the other?

fwiw, I've built apps with both, I prefer Django, But I wouldn't agonize about switching to Rails for a project or two if required. And no matter how much advice you get only you can decide which one suits you better and you can decide that only afer using both in serious projects.

Just start writing code.