Where are you staying?
I was on a Road Scholar tour and spent 6 nights in Aix (day trip to Marseilles) and 6 nights in Arles.
I loved Arles and the juxtaposition of the Roman Ruins and the Van Gogh stuff in Arles and the museum there is excellent. Just walking along the river was pretty neat as well.
Aix was fine, filled with university students, a big food area. 2 nice museums. I learned a lot about Cezanne and appreciate him more now that I've been there and saw what he saw. Marseilles...hmmm...just did a day trip here, big city, not my favorite location. Cathedral was interesting but our guide didn't mention until we were leaving the relevance to WWII and I will admit to being remiss and not studying up on it first.
I guess it depends on what you think you might want to get out of each AND where you are coming from to get there.