I use and love my waist harness, have only used a seat once or twice to see what it was like.
They're much of a muchness I think, the most important part of buying a harness is to get one that fits. Try on every different model that you can in the shop, hang off the practice loop, twist around and try to get the harness to ride up (for a waist) and try to make the straps crush your man-package (for a seat). Buy the one that is the most comfortable in all aspects of movement.
Also don't be afraid to try the kiting harnesses, I use a kiting harness because it was the one that fit me best in the shop. My personal opinion is that kiting harnesses are built stronger because kiters use the harness more, but others might not share that opinion

If you do get a kiting harness make sure that you don't get a 'hammerhead' hook, this makes it hard to unhook when you're gasping for breath and twisted up underneath the sail after a hard catapult