Base in Arles or Nimes?

I've stayed in each for over a week. Trains don't work from every place to every place so you should check also bus availability.
I missed a bus in Aigues-Mortes and ended up taking a very expensive cab back to Nimes. A hotel room would have been a lot cheaper but I was meeting people. Both Arles and Nimes are favorites for me so I couldn't chose one over the other. Maybe a good idea would be to base your choice on transportation options and availability.

75020 - will do!

I had the exact same choice: Nimes or Arles, 5 days without car. I chose Nimes — for late May I found a lovely room in a house with pool for $45/night. I plan to daytrip to Arles, Pont du Gard and Uzes. You don’t say when you’re going, but if useful for you I can post when I return.

"Initial plan is to take the train to see places like Nimes, Arles, Avignon, Aigues Mortes, Sete, Le-Grau-du-Roi ..... ."

From a purely logistical point of view Nimes is more handy as hub for these towns.

Thank you all for the feedback. I am leaning towards Nimes.