8 nights in Provence-two bases?

I would choose Arles and Roussillon.

Matt

I'm in the reading-guidebooks phase of prep for my upcoming trip to France, too, though I land in Nice on May 1. It's my impression that you're early enough to have no significant issues with off-season closures. I do see that a lot of sights have opening hours (and sometimes opening days) that are highly influenced by the calendar. September is sometimes grouped with earlier months as part of the peak season, but not always, so you may find sights that would have been open straight through the day but are now, during your travel period, closed from noon to 2 PM for from 1 to 3 PM. If you have time, I think it would be worth your while to Google the sights you want to be sure not to miss. It's usually quite easy to identify the official website, where you can find what one hopes is accurate information on visiting hours. Be especially careful about closing days. A lot of places are not open 7 days a week.

I don't know whether you've had time for much research on Barcelona, but there are a number of sights there for which you'll want to buy tickets ahead of time (not necessarily super early, just don't expect to walk right up, buy a ticket, and head inside. Lines at the most popular sights can be very long, and you don't have a lot of time there.

In Provence, i'd say it's very possible to only have one place too stay and be within let's say, an hour of everything.
No packing unpacking to do.
If you are just south of Avignon, St-Rémy for instance, you'll be able to reach the followings within an hour
Pont du gard, Nimes, Uzes, les baux de Provence, Roussillon, Senanque abbey, Gordes, Arles, aix-en -provence, Orange, vaison la romaine, glanum, Avignon
Under 1 1/2, would be Cassis, mont Ventoux, gorges de l'ardeche.
And St-Remy's area is wonderful.

I think both Arles and St Remy are wonderful choices. Arles a bit more lively at night, St Remy easier to get in and out of by car.

With 8 whole days, I don't think it is unreasonable to think about a base near the Rhone plus a base in the Luberon. Claude is right that things are not so far apart to require it. But if the idea of mornings and evenings in the hills appeals to you, then you might reasonably choose to spend the night there.

Have a great time!

We were pleased with L'Isle sur la Sorgue as a 7-night base in May 2016. It was central for the Luberon villages, as well as day trips as far as Cassis and a Cote d'Rhone villages/winery visit. It's large enough to provide ample restaurant options, and the Thursday and Sunday markets offer more than you'll need for the eating-in nights. Echoing others, we enjoyed St. Remy one morning and would short list it as a base if we were to return. If you put some of the suggested towns into the "Search" box above, you'll find tons of comments -- pros and cons -- on most of the options. I think it's hard to make a mistake in the region; pick something that suits your tastes...and don't wait too long to lock in lodging.

BUT: a side trip to Cassis is not worth the bother, too crowded, extremely difficult parking and circulation, not that pretty. Rick here gives a one star attraction three stars

I have to disagree with Tom, this is just a personal opinion.
Yes it can be tough to find parking, but the small port is pretty and the baot tour to see the calanques is memorable.

Yes, just a personal opinion about Cassis, but also consider the time involved, a good 2 hours from where most people stay in Provence, and there's quite a bit better to do that's closer. We did the calenque hike, not the boat trip, and the hike seems to be mostly through abandoned parking lots.

Stay in Arles & visit by train ...
•Nimes
•Avignon
•Aix-en-Provence

Stay in Arles & visit by car ...
•Les Baux + St-Rémy
•Pont du Gard + Uzes

Stay in Roussillon & visit by car ...
•Orange + Vaison la Romaine
•Gordes + l’Isle sur la Sorgue