Having done all three of the tours you're considering, here are my opinions
Behind the Seeds: We really enjoyed this a lot more than I even thought I would!! There were a lot of cool facts and interesting information shared, but my most favorite part was walking around in the greenhouses waving to people who were on the ride
It was really neat to see all of the behind the scenes things, but also a close up view of the little signs, etc. We got to feed the fish in some of the tanks, too. Definitely worth the cost, though it's closer to an hr and half to two hours. I definitely didn't plan for that amount of time forcing us to race across the park in order to not miss our Frozen Ever After FP...oops!
Marceline to MK: My Mom and I did this tour in December 2016. It was about 3 hours, which was perfect. We rode Carousel of Progress and Haunted Mansion during our tour, as well as went backstage behind Fantasyland. I loved hearing the stories about Walt, even though I was familiar with many of them, while walking along through the Magic Kingdom. I HIGHLY recommend this tour, especially if you're wanting to do a tour without spending an entire day on it. The cost is more affordable than KTTK, too.
KTTK: Rob and I did this tour in January and absolutely LOVED it! It is basically a 5 hour version of Marceline to MK tour--there isn't a ton of repeat in the information or stories, which I really liked. There were a lot of secrets and tidbits pointed out that even a veteran Disney lover/goer may have missed before! The highlights of this tour for me were going backstage in a new area, seeing the Once Upon a Christmastime and Festival of Fantasy parade floats, AND of course, THE UTILIDORS!!!!! Lunch is included in this tour, as well. Even though we spent a good chunk of our day on this tour, I didn't feel like we were cheated out of park time because we got to experience some attractions and had plenty of park time left following the tour. I'd recommend getting on the first or second tour of the day!
Hope that helps
If I can clarify anything, let me know!