Mike
We used our ACSI card for the first time last year at Verdun, nice site and would like to re-visit. Personally I would use a mixture of both which in France gives you, I think, a choice of around 800/1000 campsites. Given that the ACSI card costs less than a fiver it seem silly not to take one. Obviously with Camping Cheques (which we use a lot) you have to pay for them upfront before you go regardless of whether you use them.
David