I've done both- the Keys to the Kingdom (Just MK) about 10 years ago and my husband and I are doing the tour again next week. It's about $60 but you can get a 20% discount if you use your Disney VISA- better than the discount for passholders (15%). In this tour, you get to go in the tunnels and see plenty of the backstage magic. Something that I think may have changed is that they now include lunch at Harvest House. (this wasn't included 10 years ago, so I'll have to let you know what happens). It's about a 5 hour tour.
I've also done the 3 park tour (I believe it's the Backstage Magic tour), which is about $200- which again you can get the different discounts for. It's an all day tour, but very eye opening. When I did it, it included lunch at Mamma Melrose, but they changed it to Whispering Canyon at Wilderness Lodge. You get to go through Epcot (living seas, american adventure, etc), Hollywood Studios and MK. The MK section isn't as detailed, as the Keys to the Kingdom, but it's still interesting.
Depends on how much you want to spend, but for Disney fans, they are both interesting and give you a different appreciation for Disney Magic.