We use both. This year we had five days using ACSI at Les Rivages near Millau, saving 7 euros per night over the normal site price. We then had five days at Le Boucanet using Camping Cheques, saving nine euros per night, then four nights at Domaine de la Faurie in the Lot, saving four euros per night. This adds up to a significant amount over a two week holiday - and both are equally convenient for us. If we buy too many Camping Cheques then we use them in England (usually Rivendale or The Quiet Site in the Lakes) or take them with us next year.
We can book Camping Cheque sites in advance if we wish, as we have a Gold Card - but usually don't, as outside peak season we've never found the need - and no advance booking means you can simply move on if the weather is bad, or if you just fancy a change of site or area.