Haven are good because you have the pool and kids entertainment
So does Butlins, but at Butlins there is loads of other entertainment included in the price (and the pools are better). It has been a few years, but the Just For Tots weeks had indoor and outdoor fairground rides, soft play, playdough sessions, meet exotic creatures sessions, show about dinosaurs, and the opportunity to see performing on stage and then meet a good number of their favourite characters (ie people in big furry suits).
We camped at a Haven last summer, one of the bigger newer ones, and it was a mistake we'll never make again: we felt fleeced at every turn. We had picked it due to location and thinking it would be nice to camp somewhere with good facilities and entertainment for once.
Everything was an extra except the postage stamp-sized pool (which was a pain to book and do if you wanted to swim daily), tiny playground, and evening entertainment. Butlins entertainment had pantos and kids' celebrities for their evening entertainment, whereas Haven had pot luck performers (two were good in our 10 day stay), a lot of quizzes, and on one particularly awful night where clearly the entertainment had cancelled last minute, one of the blue coats singing out of tune karaoke for an hour... The paid-for activities were expensive for what they were, and to run salt into the wounds, we found the timings pretty dishonest eg paid £12 for 45 mins on the climbing wall, but when we turned up there were only 3 walls and 12 DCs per session. Once the harnessing had taken place, they were lucky if they had 10 minutes of actual climbing. In comparison, their usual indoor rock climbing centre charges £15 for a whole hour of climbing, it has more walls than DCs per session, and the kitting up and safety briefing takes place before their hour starts. We found the timings for all the activities at Haven worked a similar way, but the climbing was the worst example.
On top of that, the drinks in the hub were really expensive and the campsite etiquette was awful. Think multiple pitches blasting music, and if you had the audacity to politely ask the pitch next to you if they could just turn it down a little (not even off, just not at full volume less than a metre from our tent), you got an earful of delightful language combined with an colourful explanation that this is what is to be expected at a Haven (thank you: noted). If you can't tell, we were not impressed!