I went past Kenilworth Castle in a car on the way to somewhere else many years ago, and didn't realise you could actually go there and see something...it's now been added to my list!
As far as Warwick Castle, I think it depends on the types of activities that they have going on there. It's been more than five years since my mom, my uncle and my kids and I visited - we had a great time, but we weren't so much impressed with the interiors of the castle as what was going on in the grounds.
There were actors or re-enactors there, demonstrating all kinds of arts, like the hand-making of bows and arrows. There were also two men who demonstrated what went into 'dressing' a knight to get him ready for battle - and then they had a little battle once they were all dressed up. Very educational and entertaining at the same time.
My favourite spot was the mound - the remains of the original 'fortress' built by William the Conqueror. From the mound, a beautiful view down on the Avon....
It's not a place I'd visit again, but we enjoyed that visit...