You will notice that, as ever, the respective schools are promoted by their current students who, almost by definition, have no experience of the schools they didn't attend. Therefore their opinions are, not to put too fine a point on it, worthless!
Almost all of the groundschools that are used by the contributors to this forum are worthy establishments that try to do their very best for the students who pay their mortgages. None (as far as I am aware) will try to give you less than good value for your money. All you can do is visit those places that interest you and, having talked to the staff, make your own mind up as to which will suit you best. Don't ask the students of other schools to make your mind up for you; that way lies dissatisfaction.
Scroggs