Nimes or Avignon

Nimes. Lots of interesting stuff. Many Roman sites. A lovely public garden, a museum of "Vieux Nimes". with a great exhibit of textiles. Nimes was once a center for textile manufacture; the word "denim" comes from "de Nimes". Avignon, on the other hand, has a half-destroyed bridge and a papal palce, now mostly empty.

My first choice would be the Pont du Gard. Have you been there?

I like both Nimes and Avignon. I would choose Nimes for Roman stuff. I really enjoyed Avignon. I love medieval history so I really enjoyed the Papal palace, and the bridge may only be a partial bridge but it was built in 1234 and was used almost 400 years, which I think is pretty incredible.

You may be able to see both of you stick to the major sites.

You must be skimming over places if you don't have enough time to include both. I'd be looking at the wider journey you plan to make with the car.

Avignon is interesting, but are you spending the night there? If so, you do not need your car to enjoy it.
If this is part of a road trip I would choose Nimes, or better yet, another smaller village. Hard to advise without knowing what else you are doing.