Bandol or Cassis

I'd say Cassis, since I didn't go to Bandol ! But I found Cassis very charming and lovely for the holidays. I stayed there : http://www.petitfute.uk.com/accommodation/hotel/le-jardin-d-emile/e-233312.html I was perfect.
and if you choose Cassis, you have to go eat there: http://www.petitfute.uk.com/restaurants/french-cuisine/la-place/e-266071.html

I would think Cassis as it is close to Marseilles where you can make connections, but I'm not knowledgeable about public transport in the area. I suggest you post your question on Trip Advisor Provence forum to get advice from local experts. We stayed a night at Hotel Liautaud on the picturesque Cassis harbor. The location is perfect, the hotel far from it, so I advise that you stay elsewhere.

Where do you want to go from your base? Both towns are on the local train stops. It's a few minutes' hike or bus ride down into Cassis from the station. There are also bus connections from Cassis to Marseille and Aix-en-Provence, among other places. Cassis has a more dramatic setting and the famous calanques and cliff the hike.

Thanks for the info Tex. I did go onto the Forum on Trip Advisor and it is about 50/50 for Cassis and Bandol.