La Ciotat or Cassis as a Base?

nsbtraveler,
If I recall correctly, La Ciotat is more of an industrial town (Power plant? Not sure.) Cassis more residential and tourist-oriented, close to the calanques if they interest you. But I wouldn't risk any money on my memory. It might be worth checking out.
For me, I prefer Bandol...lovely harbor, small but nice sandy beach, waterfront cafes, nice open market, nearby wineries. (Also, Sanary sur Mer nearby is nice.) Easy drive to La Ciotat from there. W/O a car, there are buses, but I can't speak to their frequency. There are winery tours of the area available.

The location of the train station in Cassis might give one pause if side trips by rail are planned. It is quite a distance from the harbor area where the sights are, and the walk from the harbor to the train station is most distinctly uphill.

Google Maps gives you a hint about the situation. The distance from the train station to Plage du Corton is 2.1 miles, which Google would normally estimate as a 42-minute walk (20 minutes per mile). In actuality, Google Maps' walking-time estimate is 55 minutes. That 13-minute difference is a reflection of the uphill slog the walk will be. I did it at age 65 without great difficulty, but it wasn't fun, and I had no luggage. You could run in to significantly hot weather in July, which could make that daily 4-mile round trip unpleasant.

There's a bus that runs up to the train station, but I don't think it's very frequent. At the very least, I'd dig for that bus schedule online and compare it to train departures and arrivals likely to be key for planned day trips.

I do not know La Ciotat at all.

If you tell us what other places you hope to see while staying in that area, perhaps someone will have a practical, alternative base for you.

Neither is great without a car. In both cases, the train station is out of town. La Ciotat is a much bigger town with OK bus connections, and as a town, I much prefer it to Cassis (where I find the village to be of limited interest), but the scenery and hiking access is much better in Cassis. So, for 2-3 days, I would choose Cassis.

Also, regarding hiking in July: it is HOT and sometimes the hiking areas are off limits due to forest fire risk. They've improved the risk management policy so there are now fewer days when the Calanques are closed, but it does happen.
There are NO water sources anywhere out of population centers, by the way, so for a day hike in the Calanques (or anywhere else in the area) in July, you'd need to carry 2-3l of water to be safe.

We spent six months in Cassis. Our hiking group stopped in early May. From May on, other hikes we did began at 6 or 7 am to avoid the heat. As balso said, some of the areas could be closed if there is a fire hazard.

Thanks to all for your responses!