Provence - Luberon or Côtes du Rhône?

And leave us with one more day to explore Luberon region. It seemed so beautiful.

Yes but how many days do you have? One day to explore a very large region? Anyway, the obvious-third trimester pregnancy, small child, COVID endemic. You’re sure about this??

Rick Steve’s suggested one day for Luberon. We have 3 full days in Provence.
I’m exiting my 6th month while I’m there, technically still second trimester. My baby is nearly 2. Yes, we’re very sure… we are going.

The Luberon is a large region with many small towns and villages. The town of Lourmarin is within the Luberon and makes a good base as it’s flat. But to explore the region at all you’ll need 4 days to a week minimum. All due respects to RS but one day? No way.

I haven't been up north as far as Vaison La Romaine, as we stayed in Pernes Les Fontanes and did various day trips when I was 5 months pregnant. I think anything you pick will be pretty and you will enjoy being in the area. That said - how much time do you want to be in the car? Arles/Camargue will be an 1+ hrs each direction from Marseille, Marseille to Nimes is 1h40m and goes through Arles. Is your night between the Camargue/Arles day and Nimes/St Remy day in Arles, or all the way back to Marseille? If going back to Marseille, that's many hours in the car. I personally wouldn't find that very relaxing with a 2 year old and being 6-7 months pregnant. Luberon villages such as Menerbes and Lacoste are smaller towns, and would be very beautiful to visit. But they are still an hour drive for you if coming from Marseille. Obviously everyone has different preferences with how they spend their time, and sometimes the drive can be very enjoyable as it's such a pretty area. So, just some things to think about. Are you budgeting any time for Cassis? The Calanques are so beautiful, and much closer to you in Marseille.

We've done both.

Luberon is more compact, 20 minutes from one end (Gordes) to the other (St-Saturnin-les-Apt), although you could go farther if you wanted. One day was more than enough for us to see Gordes, Roussillon and Menerbes. Here the object is to see the small villages (try to visit on market day), mountain views, and lavender fields. Just to the west of the Luberon, go to L'Isle-sur-le-Sorgue, our favourite Provence town (its a rare oasis in otherwise dry Provence, the Sorgue river breaks into several rivulets that pass through the town making it a real "Venice of Provence"; we also loved tubing down the Sorgue river from Fontaine-de-Vaucluse to ISLS. Luberon is closer to your base in Marseille.

Cotes-du-Rhone is larger, especially if you're going as far as Vaison-la-Romaine, which I wouldn't really call Cotes-du-Rhone anymore. The object here is wineries and wine towns. At the Pope's Palace in Avignon we enjoyed a nice wine tasting and learned about all the different wines in the CDR region. I frankly don't remember it being as scenic as you perceive it, but we were mainly on the autoroute to be fair.

Me, I would rather do the Luberon / L'Isle-sur-le-Sorgue again.

Parts of what you call "Côtes du Rhône" are quite pretty (especially around Séguret, and Vaison-La-Romaine), but not quite as scenic as the Luberon, in my opinion.
Also, there is more hiking in the Luberon, as far as I know.