DH took DS to Hever over the summer and said they have put in a lot more stuff for children over the last few years and DS would happily have spent the whole holiday there. So I'd probably vote for Hever.
We have been to both and spent enjoyable days at both. We particularly enjoyed the water maze at Hever. From what I can remember more activities at Leeds. Watch out for the swans though if you have a picnic!
Have been to both this summer. DCs prefer Hever Castle. Tho part of that may be because of the water maze, which will be closed at this time of year. I think they are also doing some sort of Halloween activity this half term.
However, Leeds castle is also very good, and easier to reach as it is right off the motorway. Good play area too, and a nice long walk thru the grounds.