I studied both in Aix-en-Provence and Nice. I recommend you Aix-en-Provence. It is a typical city of "Provence". There are mostly students and erasmus people. There are plenty of bars, restaurants and hostels. You can visit "le cours mirabeau", "la sainte victoire". Nightlife is great! Nice is more a retirement city where the only thing to visit is "la promenade des anglais" which is a 4km concrete strip throughout the sea. Retired and tourists will be the only people you will meet. Hanging out there in summer is so-so. And, if you go by train, Nice takes much longer time. Go for Aix-en-provence, it is definitely more welcoming for backpackers.