We did not spend much time in either of these places, and Arles was quite literally a quick photo-stop on a tour that took in Arles, Orange, the Pont du Gard and a pony trek in the Camargue all in a day trip starting from Valvigneres.
Anyway, that day was mainly a roman day. Following the A7 towards Orange at first, where we took in the Triumphal Arch and the Roman Theatre, before taking the A9 out of Orange towards the Pont du Gard (has its own journal).
Next stop Arles is a short drive south east from the Pont du Gard, we stopped briefly here to get some external shots of the Roman Theatre, before we followed the road south west of Arles towards Saintes Maries de la Mer, taking time on the way for an hour's Pony Trek in the Camargue. Arles itself is literally on the edge of the Camargue.
Going back to Orange, I'd say this is in many ways underrated, as I find it tends to get overshadowed by Arles and Nimes. In addition to the impressive arch and semi-circular theatre (Emperor Augustas taking center stage mounted within the main wall), the town itself has style and charm, most obviously in and around its main square in the center of town with its pavement cafes and shops surrounding it. Well worth a couple of hours - especially if the sun shines.
I'd recommend this itinerary for anybody, but the best advice I could give would be to give it three days, not one. Take more time than we did and do the sights and the towns more justice. Take in Avignon and Nimes too. These were definitely opportunities missed. If my memory serves me correctly, I think I chickened out of visiting these towns because they are bigger than Arles and Orange and I couldn't be bothered with the traffic problems that generally go with driving in big towns.