Return rental car in aix?

Where are you going afterwards? And by what mode of transport?
Are the return locations convenient for onward travel?
That would be my criteria, whatever makes it easier for where you want to visit and your onward travel.

I will have been driving and staying from nimes to Aix over 5 days
And dropping sisters off to either aix or marseille for tgv train to paris.
I am staying in cassis relaxing for my last day and a half before going up to Paris.
Im leaning to dropping car, taking train to cassis and then walking around and relaxing in cassis and then getting on tgv in marseille (no car) or Aix (with a car). I have heard that marseille rental return can be difficult to find ...wondering if its easier to drop at Aix?

My guess (only a guess) is that the new Aix TGV station, which is outside of the city, will be easier generally than the older St Charles in Marseilles.

You could find the location of the drop off for each on the internet map search of your choice, and see what looks easier for you.

We've never rented a car in these cities but we have driven in both and been at the train stations in both and the Aix train stations, both TGV and regular train station, are less hectic than the train station and driving in Marseilles. We also enjoyed driving from Aix to Cassis for some seaside time.