I like the look of the waist harness better, but there are some seat harnesses which are really sick, the dakhine one comes to mind. Toby has a good look too, he has a black waist harness with black shorts. I don't like the "diaper look". I mean go for it, if it's your thing, but that's the number one reason I think not to wear any old seat harness. I also love the look of the ride engine waist harness (it's what I'm using now). I ride my harness with about 1 or 2 inches slack. To keep it from riding up I will bring the kite down low. The slack gives you a bit more give on a bad crash not as bad for the ribs.
So much about kiteboarding is "style" is your style wakestyle, airstyle, surf, hydrofoil,foil kite, inflatable, what is your style? Part of having style is deciding on waist vs seat. There's no wrong answer. Ride with style, and throw down that's all you have to do. Create a style and make it your own. For me that means waist harness.
If you are worried about your ribs using a seat harness is a great answer. I don't think there's really an good technical reason to prefer one over the other. It really is about style I think. You can unhook perfectly well on a seat harness.