Epcot has innoventions, test track, mission space, the kidcot stations; any my kiddos love journey into imagination and seas w/ nemo- esp. turtle talk. There's a lot of characters, too.
DHS has toy story mania, and if you time it right, mater and lightning for meet/greet; muppets 3d, star tours and playhouse disney live on stage.
If my kids have to pick just one; they pick Epcot. But really, it depends on what your kids are into.