I took a trip to this area without a car for 10 nights, split as follows: 2 nights in Marseille, 3 Nights in Arles, 3 Nights in Avignon, and 2 Nights in Aix. I am glad I had the experience of staying in the different cities, and on a trip like yours (5 nights) you can certainly stay in two locations if you want.
I personally preferred Arles to the others, and like Nimes better than any of the cities I stayed in. I didn't care for Aix, and it's less convenient for your listed destinations, as it's at the edge rather than the center of them. Although I loved Nimes, it's also not as central for transit. In terms of transit connections, Avignon is the clear winner if you won't have a car (with a car it's a pain, and I'd recommend looking at a smaller town).
So, if you want only one base, pick Avignon (unquestionably more convenient for your listed destinations) or Arles (to me, nicer than Avignon); if you don't mind two bases, you can spend some nights in each.
Note that to get between Avignon and Aix, the bus was much more convenient than the train. The bus was direct, while the train would have required changes or bus connections from the TGV stations. I'm not sure if this is still the case - check.