Arles or Aix?

Having been to all 3 of your cities, I think Aix would be a lovely second location for your trip paired with Avignon. I love the beauty of the fountains, architecture, trees, markets and Mont Ste Victoire nearby. The museums and Cezanne’s studio are nice to visit too. We loved the food options, being so close to the Mediterranean.

Don't go by pictures but you should try Aix since Avignon where you are staying already is an easy ride day trip to Arles.

From Aix you can take a commuter train to the Vieux Port of Marseille, and an international airport. There are many World Class Museums including the Museum of European & Mediterranean History Civilization. There a plenty of great places to eat. I enjoyed a voyage to the island prison of Chateau D'If made famous by Dumas' Count of Monte Cristo.

I'd vote for Aix as well. Really, I'd vote for Aix and Arles, hahaha but you brother can have his choice. I stayed in Aix for 6 nights and Arles for 5 on a Road Scholar tour and they felt very different to me.

And still another vote for Aix. I have visited all the places you have mentioned and would choose Aix every time. Its a wonderful town and there are several possible day trips from there if you run out of things to do (doubtful!).

What would you recommend as a day trip from Aix?

Marseille is a great day trip from Aix. Very easy to do, buses run very frequently throughout the day and evening (there are also trains, a bit slower and less frequent but more comfy).

If you are fit hikers, you could hike up the Sainte Victoire and feel like you are in the middle of a Cézanne painting. Buses serve the trailheads, and a cab won't break the bank either. Do inquire locally about trails etc. and get a map - as a starting recommendation, the Pont de l'Anchois - Refuge Cézanne - Pas de l'Escalette - Croix de Provence trail is a safe bet (steep in parts, but no climbing involved).

Lourmarin, a village in the Luberon, has direct buses from Aix (1hr) and is very worthwhile.

You could do Nice also from Aix. We stayed in La Seyne Sur Mer (near Toulon) for a week and easily did Marseilles, Aix, Arles, and Nice from there. Separate days of course except we did Marseilles for Calanque boat trip and Aix for Cezanne in one day. If I had to pick between Aix and Arles, I'd have to go with Arles. The old Roman architecture makes you feel like you're in both Italy and France! But I'm also a huge Van Gogh fan, so there's his history there to discover too.

Aix - In addition to the museums, there are market tours and cooking classes to be had. Or consider a small group tour to the villages of the Luberon such as Roussillon, Gordes, Isle-sur-Sorgue. We stayed here for a few days before taking the train to Avignon for a river cruise. We wished we'd had more time. If you want to fly out of Marseilles, it is about a half hour bus trip to the airport.

If it's not too rude to piggy back on the OP's question, I'm staying in Aix in mid-July for their music festival.
Is a day trip to Pont Gard and/or Arles easy to do w/o a car? Perhaps with a small tour group or guide?

Unlike Avignon, where there are almost an endless number of nearby visits, Aix-en-Provence does not have a great deal nearby. Sure, you can go to Nice, but why not just spend 4 days in Nice. Marseilles has some interesting museums, but I really would not recommend spending a lot of time there.

Consider spending 6 days with an Avignon hub, then 2 in Aix. Isle-sur-la-Sorgue, and the villages of the Luberon are much easier to visit from Avignon, as is Arles, Nimes, Pont du Gard, you can even take the train to the beach at le Grau du Roi (it's cold in March, but the seafood restaurants are fabulous).

Todd - you can visit all the places you mention by first taking the train to Avignon Centre.

While Aix is sophisticated and lovely, there isn’t much nearby for side trips. (Marseille is skipable and Nice is too far away to give it justice as a side trip.) I suggest you stay in Arles and visit the Camargue (with its flamingo park, white horses, and special black bulls). The two towns in the Camargue (Saintes-Maries-de-la-Mer and Aigues Mortes) are both very unique and interesting. Saintes-Maries holds French-style bullfights (no spearing of the bull) that are delightful and Aigues Mortes is a typical Bastide town built on a grid and with salt flats just outside the walls. Something to consider!

We’ll be in Aix late April. We’re excited about a David Hockney exhibition at the Musée Granet!
And Cézanne’s studio. Plus the rosé tastings.

We loved Aix, a mini Paris, in the south of France. Loved walking up the hill to Paul Cezanne's studio up the hill, for a look, and little tour through his place of painting and garden. Objects he used are on display. It is a charming place to soak up French life out of Paris.