Hi there! I'm glad that you have decided to go to France for a year abroad!
Personally, I heard good things about Toulouse, so I think you might want to consider that as an option. I have also heard about the high crime rates in Marseille and I don't think it would be a good idea to go there unless you believe that the advantages outweigh the disadvantages (if you really want to live close to the Mediterranean - I bet it's great!)
Currently, I am doing my international exchange in Sciences Po and I must say that it is not bad. Like you said, it is not exactly the best French institution for law (in terms of Paris - that would go to Sorbonne or Assas), but it is located in the centre of Paris! It is true that Paris is an expensive city, but if you live frugally, things just work out fine!
I don't know much about Paris-Sud 11, but I would assume that it is located in the suburbs, so you will be located pretty far away from Paris. This might be advantage, given that it is cheaper living away from Paris yet being only a stone's throw away from it.
All in all, I think overall, Toulouse would be your best choice, since it is in southern France without the crime rates. However, if you really prefer the nice weather, then undoubtedly Marseille would be your best choice. For a nice lifestyle located in the heart of Paris, Sciences Po would be the ideal choice, and for a university located in the suburbs with close proximity to Paris, Paris-Sud 11 would be the one to go for.
NB: I just found a list which ranks French universities for law. It's kind of outdated, dating back to 2007, but you can get a general gist of it (if you are concerned about the reputation of the university that you will be going to).
Here's the link:
http://www.devenir-avocat.fr/choisir-fac-avocat.php.
Hope that helps!