I am using both, for slalom seat and for waves or high wind waveboard freeride waist.
I have a small problem with waist as it travels up on my body, but in waves this is notthe problem since you are not hooked that often and you do often hook or unhook --> it is than more easy, faster and kind of sure.
The seat harness, I like on slalom or freeride blasting, since you can have bigger sail, you put the weight on it so that you let your bottom go down and the upper part of your body still is in the best position relatively to the boom (and the harness does not go up on my body). I can sail with my arms streched (so i can hold a bit bigger sails).
I kind of like both of them for different type of sailing. hardly i can say which one is better.
PA 86 & eVo 100 (with S-1); iS 101 & 122 & 144 (with Overdrive)