It depends on what your experience you have. If you know Python, use Django. If you know Ruby, use Rails. I know Python, so I use Django. Since you have no experience with either, this one is moot.
It depends on what you're building. A lot of people on the net seem to take the position of "Django is good for content publishing sites, and Rails is good for web apps", but that has more to do with the applications the two frameworks were derived from than the frameworks capabilities. Realistically, anything you can build in Rails you can build in Django, and vice-versa.
Personally, I love Django. It lets me develop quickly and I know how to deploy it properly. I tried setting up Rails no more then three days ago (on Ubuntu), and it wasn't as much fun. I had issues installing the correct version of Ruby, and then Gems didn't want to work.
As for you, I'd recommend spending a few days with each framework. Get them both installed and just tinker around. Build a quick CRUD app. After that, you'll have a pretty good idea of which one suits you (and your needs) better