I've never done KTTK so I can't speak for that one, but I really enjoyed the Marceline to the Magic Kingdom Tour.
The most obvious difference immediately is going to be length and cost. KTTK is five hours, and twice as expensive. Marceline is three hours. KTTK includes lunch, Marceline does not.
I'm not sure the age of the guests traveling with you, but you have to be sixteen to go on KTTK, Marceline accepts guests as young as twelve, as long as there is an adult eighteen or over with them.
Marceline includes a lot of walking, a lot of talking, and a limited amount of rides, with the tour guide giving you extra information about them. While I found it very interesting, I probably wouldn't recommend it for children, or anyone who doesn't have a definite interest in the history of the parks, the company itself, and Walt as a person.
My take on it? If you're already wondering if you would find it too long or boring, and if you're afraid you would spend the time wishing you could be riding the rides, eating, or shopping, then you're probably right to be concerned, and it may not be worth the cost to you. If on the other hand, seeing a little more of the inner workings and backstory of the park appeals to you, you may very well enjoy it.
It's probably not something I would recommend to first time visitors, but if you're looking for something 'extra' for you trip, each tour I have done has always been interesting, engaging, and informative.