I think I like the second option better as well.
If we are going to require an option to turn the Customer models on,
then it seems reasonable to require another option to indicate which model.
Also, django 1.5 has custom user models anyways, so you can't just hardcore auth.User.
You can also combine to options, i.e. if setting 'AUTH_CUSTOMER_MODEL' is not empty,
then include customer models, otherwise don't.