There are quite a few different Haven centres and they have quite a range of facilities, depending on which park you go to. It is probably worth having a look at their website to understand the differences. For example, the centres at Pwllheli and Ayr used to be Butlins centres, so they have Butlins style large swimming centres, whereas some of them are smaller, more laid back parks. I've been to a few of their parks, do you have a particular location in mind?
I would agree that it is probably worth paying the extra for a higher grade of caravan, for instance although there is heating in the main room in all grades, there is heating in the bedrooms only in the higher grades.
Haven is owned by Bourne Leisure these days, who have a good record of investing in their parks each year so they are generally very well kept.
No matter how cheap it is, never ever go to a Pontins centre. Never.
We run our events (or have done in the past) at many of these holiday centres so fire away if you've got any specific questions and if I know the answer I'll try to help.