UC is pretty much right about the restraints. Kingda Ka's restraints really allow for the airtime it has, and they don't really touch your body, but they give you that feeling of being very safe and restrained.
As for Top Thrill Dragster, it doesn't have an airtime hill so its T-bars, though they don't allow for airtime, suit the ride pretty well. Though they can make your waist feel pinned in, you can always leave a little space, and hope the ride operator won't push it down to high heavens. Also, it doesn't make your upper body feel confined.
So UC pretty much hit the nail on the head, but I was just backing it up and adding the point that Stealth and KK's restraints suit their type of coaster (airtime hill), and TTD's restraints are suited for its particular model (no airtime hill).
However, what REALLY pushed Ka over the edge for me was the speed hill. I didn't realize how much I liked it until the last time I went, but flying over that hill at 100+ mph in the front row is absolutely amazing.
That does seem quite fun, but I've heard people say that going 120 MPH on a straight stretch of track is better than the airtime hill, because it doesn't show you down as fast and you gain a good feeling of speed. I don't know how much I believe them though, but it might be true.