In Django, is it better to add admin features to the command line or the admin interface?
I sugest You to stick to the web interface if it will be used by a human. CLI is good when You want to control Your application by another app or some sort of script.
Many people could have a problem using CLI. Remember that potential client will almost always be 'less technical' person than You, so the UI should be user friendly for him.
In my specific case, I'm building a relatively straightforward email-sending webapp. I'll need to add the ability for a human to generate email templates from the last day's worth of updated objects, view the day's last sent emails, etc. 
Django provides the Django manage.py CLI, and also the Django Admin Web UI. Both are augmentable. Are there any best-practices or rules of thumb to follow in terms of adding my email admin functionality to one or the other?
Edit: To be more clear, my big concern is around speed of implementation operations on models and such.