Well I've been to Prairies loads of times as my mate works there as the manager for Club Cantabrica. I think it's excellent and the positioning is fantastic, especially when I compare it to other sites I have been on which have also been lovely but you need a car.
There is no pool, and I was disappointed about that before I went for the first time, but really as it's right on the beautiful beach it isn't missed at all. And Aquatica is not far away and great for a day out or two messing about with all the water rides etc.
The site is also great for all ages, not overtly aimed at young kids like so many others are (obviously if you do have very young kids then a pool might be a must for you) and another great thing is being based so close to Port Grimaud itself you are not restricted to the things on offer at the campsite when it's just a stroll away. The markets are lovely - Friday mornings I think and one other day in the week, can't quite remember which day. Boat trips over the bay to St Tropez I can highly recommend (guy called Eamonn has an office on the site and does taxi service to airport etc and various trips - the Gossip Cruise is brilliant - takes you to see all the homes of the rich and famous in St Trop bay - free booze, then a couple of hours in the town to do with as you wish and back to the site all in an evening).
Anyway, I'm not familiar with the other site, I'm sure that whichever you choose you will have a fabulous holiday. Prairies appeals to me as I'm not a fan of going on holiday to drive to places.