Both are excellent frameworks. Though, I've found Rails to be more suited for the agile developer. For the most part, you'll run some generators to get the files you need as placeholders for your code. Things will work right away, and you just build up from these conventions. It's really flexible and has a large community, lots of innovation and interesting practices are being put into Rails. It's development cycle seems faster paced than Django.
After only touching the surface with Django, it has some interesting differences. As far as I know, you don't get the schema migrations like Rails has out of the box. But you get an extremely simple CRUD mechanism for your models with the extensible admin interface, which is great for testing/managing content. The entire project is documented really well, from the Django Book to the vast amount of information on docs.djangoproject.com.
I personally prefer the Rails way of doing things. But honestly, you need to try them both to see what works for you, and since we're talking about two very good, yet totally different frameworks, it's a tough decision to make either way. So, if you already know Ruby or Python well enough, start with what you know and just go from there. Once you understand how one works, you'll be able to evaluate the smaller differences yourself. Hope that helps.