They are very different beasts. As you say the grounds at Leeds Castle are lovely. It was lived in relatively recently so some rooms are as they were then, more like a stately home than a castle as such. There is a maze, a small aivery, sometimes displays with hunting birds, places to eat, relax and walk. Dover Castle, is very different. There is more to "do". They revamped the Keep a few months ago with stunning tapestries etc to give more of a feel of Henry IIs time. It is more historical (Roman lighthouse, medieval keep, Tudor tunnels, WWII exhibition etc) and although the site might be less polished than Leeds its a big site with more to explore. Of course there are also the war time tunnels and amazing views of the channel etc.
You could probably do either in half a day but equally you could spend a whole day and not be bored
Kit
Thanks for both replies. I think it's gonna work out that we can do both - Leeds the Saturday morning of our cruise and Dover the Thrusday morning we disembark from our cruise. Both look awesome!
We've been to Edinburg Castle, Sterling Castle and Eilean Donan Castes so we're anxious to add two more English castles to our list.