Got lots of choice at either place here's what I know
La Rochelle is "Seaside" to the French
WE prefer St George de Bidounet to Royan (it's just around the corner from Royan) for eating and relaxing - think the beach is nicer there too
WE actually stay inland at Soumeras near Montendre, and drive around to where we want to visit, Royan being only 80km away.
And as we stay at Twin Lakes, I can't recommend a campsite at either place, but of the two places Royan /St George are the places
I would go to as opposed to La Rochelle
Try visiting Blaye - Castle - great Sunday Market
the town of Jonzac has a really nice tropical swimming /beach entertainment centre
also try chilled Pineau as an aperitif The White and Rose are realy nice chilled the Red is a little harsher