Hi Miki,
That question is going to be answered with lots of selection bias on this forum, and I am not sure what the "obvious differences" would be to you or others. To me, the obvious differences are that WP is a very mature dominant blog engine written in PHP with loads of drop-in themes and plugins. Mezzanine is a rapidly evolving framework for building web applications that has a few applications baked in – Blog, Pages, and Galleries – built on Django, which is a very mature (dominant?) Python web development framework.
My belief is that Mezzanine was built as a way to get content out of blog engines such as WordPress into Django. The pros for a skilled Python/Django developer are obvious. Adding new applications to a Mezzanine stack is a pretty straightforward process. Mezzanine inherits a heavy emphasis on excellent documentation from Django, which in turn inherits that same emphasis from Python. It was the documentation and tutorial, in 2006, that ultimately convinced me of Django's value. When I explore new and exciting tools today – this week, that is Meteor – I always measure them initially by the strength of their documentation when compared to Django.
If WordPress does exactly what you want and you will never need a developer to muck about with the business logic, go with it. Its like the old saying, "Nobody ever got fired for buying IBM". When the requirements are clearly in favor of a tool like WP, its hard to go wrong. WordPress is the 800lb gorilla, and you will have no problem finding cheap (and even some high quality) developers to support your efforts.
If you do choose Mezzanine, focus your energy on learning Django and Python first. Perform the Django Tutorial and create the Poll application. Mezzanine is just a Django application, albeit an excellent one.
Hope this helps.
best,
ken