Taking the two at face value, I would choose Busch Gardens. KSC is not really the kind of place where you will find enough to spend an entire day there, and after you pay for all the different exhibits, tours, movies, etc you'd be surprised how quickly it adds up.
Don't get me wrong KSC is a fascinating and awe-inspiring stop and everyone should check it out, but given an either/or option I would defintely go to Bush Gardens. BG is a fantastic amusement park loaded with classic rides and attractions (log flumes, coasters, river rapids, bumper cars, etc). It easily rivals the Universal Studios Parks IMO, and is by far the undisputed king of rollercoaster parks in FL.