We have done KTTK (02/01) and Backstage Safari (02/01, 10/02). We really loved KTTK, and although it was long, it points out so much amazing information and shows you all the hidden mickeys in MK! I also found it so cool that all the storefront window signs in MK represent Walt's "scheme" to get WDW going with out everyone knowing it!
However, our favorite of the two was the Backstage Safari at AK. When we took it in 01, they had some college interns do it, and they seemed a bit timid speaking, although the sights were amazing. In 02, Disney had hired an old HS teacher to give the tour and he does a great job. We had the lucky treat both times to be able to touch a white rhino. On the second tour we also met a giraffe and we were able to hold (only if we wanted and I sure didn't) a wee tiny alligator. I also noticed that on the second trip, the tour guide was really pushing Disney's role in conservation...almost to the point of being overdone, perhaps. Regardless, we found both Backstage tour to be informative and sooo interesting!