I've been to Sherwood twice now and it's a great holiday - there me and the wife, our kids (5 and 3), and my brother and sister and their spouses (spice?!) in an 8-berth chalet.
We'll be going to Elveden next year, as it works a little better for everyone to travel to.
My wife and sister have been to Longleat and reckon the rapids there are better, but it's more spread out (although there is a little road train to get you around).
And I think Longleat is more covered in the centre than Sherwood.
But they're all probably equal unless there's one specific feature you value.