Depends what you want to build.

Flask is better suited to apps that are a) small and single-purpose, or b) highly nonstandard.

Django has a lot of stuff built right in, much of which you won't need. But it's nice to have a database abstraction, forms, auth, admin, etc. when you need them. Plus, it's practically a CMS already all by itself, with the nice admin panel.

I'd probably start with Django. The tutorial isn't bad at all: https://docs.djangoproject.com/en/1.4/intro/tutorial01/