Hello,
These two cities are famous for their Provencal markets with local products, crafts and gastronomy. However, the market in Arles is more extensive (2 km long), but parking can be complicated.
The market in Saint Rémy takes place on Wednesday and Saturday mornings on the Place de la République, and in Arles on Saturday mornings on the Boulevard des Lices.
In Saint-Rémy-de-Provence, parking in the orange zone is free for visitors for 45 minutes and then the rate is 2 € / h, between 9am and 7pm. In Arles, parking is charged every day in summer from 9am to 7pm and free on Saturday afternoon from 3pm, on Sundays and on public holidays.
I hope this will be useful to you!