La Ciotat is bigger than Cassis, yet a small town with a good service level (lots of all kinds of mid range / casual restaurants, major Carrefour supermarket etc.). As Cassis is a tourist attraction with well maintained houses etc., La Ciotat is clearly more rugged and authentic. La Ciotat is known for Lumiere brothers, pioneers of moving pictures and the former shipyard converted to service station for luxury yachts. It is perhaps easier to find a suitable hotel there. You should perhaps do some Google Maps street view peeking in La Ciotat to understand what I am trying to say. Our intention is to have some meals and spend some lazy time in La Ciotat in the later half of June, that is in about one week's time. We think La Ciotat is at least ok, if not better than that.
In La Ciotat, there is a long beach towards east (St Cyr as the next town). The hills between La Ciotat and Cassis offer great views towards the sea and inland, hope the weather is clear when you visit the place.
If you are hesitant regarding La Ciotat, one good alternative would be Bandol, which is a very tourist friendly place, next from St Cyr, which is also OK, but focuses more on private villas and apartments rather than hotels. Bandol is well connected to the motorway, so the drive to e.g. Marseille is very easy.