Went to a Spanish site this autumn with Keycamp. All the other companies: Eurocamp, Haven, Eurosites, were all on site too. From what I could see, there wasn't any difference whatsoever. In fact, Keycamp and Eurocamp reps were working together by the end of the season.
When we went camping in France, six seasons ago, Haven seemed to have the biggest and best tents but I am sure that varies from year to year. Just check the specification of the mobile home - size, facilities, air conditioning - to see if they are truly comparable. You could always try asking the booking operator why Haven is cheaper too. Have fun booking - have the enjoyment of a holiday is in anticipating it.
We have been with HAven every year for last 8 years and been v.happy - casn tell you about several sites if you like. However, this year am on extreme budget, and found Matthews Holidays. Booked 12 nights in August on Portsmouth Cherbourg crossing with overheight vehicle (bikes on roof) for £800 (ish).This is staying in a basic van on the Vendee, walkable to beach. Would be cheaper if went Dover Calais. This was cheaper than staying in a keycamp tent on same site. No Kids clubs but mine have outgrown Rory the Tiger now.
If I didn't do it now, I'd put it off and miss out completely (as I did last year), so have actually gone ahead and booked 10 days at Domaine de Kerlann (south west coast of Brittany). This isn't a Keycamp site - but with the few extra hours to think about it, I found a Haven/Siblu site that looks really good for us.
We have been there. It is a great site - very spacious on grass pitches. Great indoor pool & kids clubs, facilities etc. The on site shop was tiny & v.expensive - may have been upgraded. We used it for crissants & then drove to nearest Leclerc for stocking up. It is worth driving to explore the beaches further away which gor bigger & more sandy the further you went - don't stop at the first one. Lots of places to explore.