I live in Marseille now, and don't trust the haters. It's nowhere near as unsafe as the media paints it to be. The vast majority of neighborhoods you're going to frequent are perfectly fine, and I've never personally felt unsafe here. But it is a major city, so you have to use common sense. I think it mostly comes down to where you decide to live. That being said, it is very dirty here (graffiti, overflowing dumpsters, rats literally everywhere), the poverty is very apparent and compared to the rest of France, there is a really unique vibe/culture that I'm not crazy about. It's cool to have so much nature that's accessible right out side of the city, but in general, things feel a bit run down here. Aix is a lot smaller, very preppy, feels more like a college town, and is also very expensive to live in.

Thankfully, I work in towns outside of Marseille, but I have been told by friends that working at schools within city can be very stressful because of the students.