Marseilles Airport Hotel or Stay in the city of Marseilles?

I would not stay at the MRS airport. There are a few airport hotels, but the general area is not attractive, I don't think not safe but very remote in feeling. I would go Marseille and stay there or go to Aix and stay there. There is a bus from the airport to Aix and I believe is only 30-45 minutes. I would make my decision based on what city is more appealing to you. I have enjoyed both. There is an article over on fodors.com today about Marseille "The City That Parisians Love to Hate Is Actually Amazing".

When does your flight arrive, and will you have the time abernathy for sightseeing after arrival? A few years ago, after I’d been in France already, I met my husband at the Marseille Provence Airport, and it was late enough that we just headed to the very comfortable, nearby ibis Hotel (https://all.accor.com/hotel/1093/index.en.shtml), had dinner, and then turned in to get some sleep. They have free shuttle to/from the airport, so that made it easy.

We were picking up our rental car at the airport the next afternoon, but took the bus into Marseille that morning for sightseeing, for lunch, and to visit the Mucem museum. We headed back to the airport to get our car, and drove to Saint-Remy, our stay for the next week.

Since you’ve got two nights before you meet the others, but will be going to that airport hotel for a meet-up in any case, which is more convenient - staying downtown and shuttling your luggage back and forth, or keeping it at the one hotel, and shuttling just yourself in for sightseeing? The bus ride between the airport area and downtown Marseille was actually pretty quick. Is hotel pricing any concern, and is one location better for you in that regard?

Artist,
I would opt to stay in Marseille for those two days. Take an Uber to the hotel you choose. Going by way of Gare St. Charles would be a waste of time, and the gare isn't in a good place for getting to a hotel. (There are hotels near there, but I wouldn't stay in that area.)
To possibly see while there: Basilica of Notre Dame de la Garde (It still has bullet holes in the walls from being bombarded in WWII. Inside are models of boats given and hung there by fishermen and their families. It offers a great view of Marseille also); the Vieux Port, where fishing boats still base from and arrive in early morning with their catch; Chateau d'If, a small island in the harbor where "The Count of Monte Cristo" was imprisoned in the book, a cool place to tour if you are a fan of Dumas; the Vallon des Auffes, a fun and charming inlet with the iconic pizza restaurant Chez Fonfon; the Museum of Mediterranean Archeology (Marseille began as a Greek outpost); the (oldest part) Panier district, and of course the calanques. There are some nice hotels in the old port area. And in spite of what they say in Nice, Marseille is the birthplace of bouillabaisse (I am in the pro-Marseille camp! Hah!) You will find some great food in Marseille, not just seafood. The Italian influence in this part of the coast is seen in the food and, if you are lucky enough to find someone who speaks it, the Provencal dialect. I love hearing it spoken, but alas, it is disappearing.
To get in the mood, watch, if you can any or all of the following films (or read the books by Marcel Pagnol): Fanny, Marius, Jean de Florette, Manon de la Source, Le Chateau de Ma Mere, La Gloire de Mon Pere. They are nostalgic, moving, personal and oh, so enjoyable! And if you can find some way of hearing music sung by a chanteur of many years ago, Darcely, you will REALLY be in the mood for Marseille!

What fun you will have in Provence! Amusez-vous bien!

I'd park my luggage at the airport hotel and go into Marseille day 1 and go to Aix day 2. The airport is between the two. There's no need to schlep luggage back and forth to Marseille just for a bed.

Thank you all for the very thoughtful recommendations!