Hi, as I see it there are two main choices - go for one of the sites that has allocated pitches, Houx, MB, Bleu Nord (and others?) and be fairly sure that your space will be there when you arrive. Or go for one of the non-numbered ones (BSJ, Technoparc, Expo, HA (correct?)) and just find yourself a bit of space when you get there.
I'd have a look on the sale and swap forum, there are still some folks juggling Bleu Nord allocations who would sell you one at cost price plus postage - we are hoping that the CA allocations are in the same area so whoever you buy from may be able to ensure that your pitch stays available for you so there isn't any problem. Allocated pitches are new for MB & BN this year so it's anyone's guess at the moment how successful it will be!