Time, flexibility, and cost would be your biggest deciding factors between those two, I'd imagine. I've heard raving reviews on both, but have only done Marceline myself, but we LOVED it! My roommate is a huge Haunted Mansion fan, so that was my Christmas gift to her during our trip last December. Marceline is a smidge over 3 hours, once per day starting at 8am and is going to be over just in time for an early lunch, whereas KTTK is something like 5 hours including lunch, but you have a choice of 4 start times I think, so if you're not super early risers, there's a little leeway there. Marceline is half the cost though, which is arguably a lot of bang for your buck. For us, the day we did Marceline, we got up at the butt crack of dawn to catch a bus from All Star Music, and after we ate lunch post-tour, we needed naps if we were going to have any hope of making it the rest of the night in the park! So we had to take a mid-day resort break, which we almost never do! Like I said though, we absolutely loved it, you spend a ton of time at Haunted Mansion from the graveyard queue to the ride and then backstage, so it's a Haunted Mansion fan's dream come true!
