This.
We have yearly tickets for both so go a fair bit. Our DC are 7 and 9 so a bit younger; they enjoy both, but probably prefer Leeds.
Personally I think I also prefer Leeds Castle - the walk from the entrance to the castle and then further on to the maze and parks is just very serene and picturesque. I prefer the ambience of the castle too - more intimate and homely; Dover is a bit more impersonal. Leeds is probably a bit more varied as well in what it offers.
But it depends what you're after. Highlights of each:
Dover: A lot of history and buildings to explore, war tunnels, watch out posts, ruins, can go right on to the roof, castle-themed educational playground.
Leeds: Very beautiful grounds, rivers and lake (which you can get a boat across), swan and duck feeding, maze, crazy golf, birds of prey, three different playgrounds.
They also both have events on over half-term. Might be a bit young for your DC, but maybe check those out to see if either particularly appeals.