I think simpler is better and think using email instead of usernames should be what you go for first. However, I do think usernames are useful for public facin

I think simpler is better and think using email instead of usernames should be what you go for first. However, I do think usernames are useful for public facing profiles in order to keep email addresses private. Another benefit is that usernames are usually shorter to type than email addresses (which is really noticeable on mobile...I should set up a shortcut for mine). Most sites I've seen accept email or username in the login field. I'm pretty sure that was the default behavior for Django last time I looked into it (years ago).