ToT drops straight down and at a faster speed, so the "air time" is greater. As others have said, it goes up and down multiple times.
There are some ride through elements before the drops which are pretty neat.
Its definintely scarier than Splash Mountain, but I say try it once and then you'll know!
For the 6 year old who wants to go, I'd let him go. But, I would also make sure he has an idea of what to expect. Dark, Haunted Mansion kind of queue and pre-show room, some dark and scary show scenes, and then the multiple drops, people screaming, etc.
That won't completely prepare him of course, but it'll give him an idea of what to expect. Kids are less likely to get freaked out if they know what's coming.