Can't weigh in on Mezzanine, and haven't built a site in any in over a year so things may have changed, but the big difference between Django-CMS and Wagtail come down to how your content is modeled. If you're looking to have a page with a variety of widgets that can be mixed and matched, then Django-CMS is probably more suited. If you've got a structured page where your users are going to be filling in predefined fields and the pages have more consistency then Wagtail is probably better.

The backend of Wagtail is completely geared towards authors, and a lot easier to use in my opinion, and it's got a media library built in. Django-CMS extends the standard django admin backend, may require a third party plugin for a media library, and tends to encourage more frontend editing, so authors will more immediately be able to see how a page looks. It also tends to be more flexible and can do more (which may be a good thing or a bad thing, depending on your goals). If you need more control over permissioning, workflows, etc. then django-cms may be a better choice.

I personally love both of them, and would recommend either.