My best answer is for you to learn both.
Consider this your end goal, and instead consider your question as, "Which one should I learn first?"
I recommend that you start with Django, especially if you have Python experience. Django's approach, like that of Python itself, is more condusive to learning. Once you've gotten your feet wet, learn Ruby on Rails, because that's the framework that will net you easier cash when you get out into the modern Web 2.0 workforce.
Once you're thoroughly familiar with Ruby on Rails and have been working for a bit, continue learning Django. Refamiliarize yourself with the basics, then start to learn some of the more advanced stuff. Django's inner guts are supposedly easier to dissect than that of Rails. If you ever find yourself faced with a job opening with a surprisingly novel and challenging web platform to implement, you'll find your Django knowledge quite handy.
Here's a bulletized rundown:
- Great documentation
- Thorough tutorial to ease you in
- Fewer files to understand at first (vs. scaffolding in Rails)
- Built on Python, which you might as well know anyway
- More similar to enterprise stuff like Java Servlets/JSP
- Easier to dig into its innards
Ruby on Rails
- It's what's hot
- Hot means more jobs
- You want money, don't you?
- When you want to make a "traditional" web 2.0 site, its generated code lets you get done really fast
- Built on Ruby, which you might as well know anyway
Verdict: Django first (do the tutorial), then Rails, then Django again