Since no one will respond, here is something.
I don't really know what the pro slalom or speed sailors typically use, but I think the majority of the Formula sailors use seat harnesses. I think the most wave and freestyle sailors usually have waist harnesses.
For the rest of us that free sail or occasionally race, we use what we have always used. For me it's a seat harness because I raced long boards for years and now formula. "Hanging" in the harness is good for light wind effeciency, plus it's better for my back than a waist harness.
I am sure there is some logic for the use of a waist harness, but I will let those that use them explain why.
Hope this help a little.