More similar than different in a lot of ways, but I suspect the differences come out of their background. Rails was originally developed by a consulting company for both client work and their own apps, so it really emphasizes rapid development -- it pushes convention over configuration very hard, had database migrations from its first version (Django didn't get those for years, etc.). Django came out of a newspaper (the Lawrence-World Journal), and seems really optimized for building custom CMSes -- there's nothing I've found in any other system that's truly like Django's admin console, for instance. (Another sign of its newspaper heritage: it's the only template language I've ever seen whose date filter has extensions to match Associated Press style!)

I used Rails for 4 years, then Django for 2 years, then back to Rails (now for another year).

Programming with Ruby and Rails is like a breath of fresh air compared to Django. Working with Django was just so painful and awkward (especially if you are trying to write tests for things). I used to have a laundry list of a couple of pages worth of things that I hated about Python and Django when compared to Ruby and Rails, but I can't seem to find it right now.

Different philosophies. I think the biggest difference is that Rails has a much bigger ecosystem and more developers than Django does. There are gem's out there that support almost everything you want to do and if there's not there are resources from people who have had to solve similar problems.