I have Naish seat and waist harnesses. I mostly ride with seat harness because that keeps my bar low all the time. No pain in ribs...
In winter, when you have to wear lots of clothes (here is has been between -35C - -20C past weeks), I like waist harness. That does not prohibit your moveability as much as a seat harness. But if it is "warm" I often ride with seat harness because you don't get wet in slash or wet snow (sometimes we have 10-20 cm water on ice with slash water).
BUT keep in mind that your body shape counts a lot. If your upper body is like figure "V" then you will enjoy waist harness but if it is more like figure "A" you might prefer a seat harness.
I prefer a high seat harness that supports my back. And other thing is that, when wind is gusty and no flat water, you might prefer a good seat harness. Helps you edge harder.
But I do not agree that you cannot use waist harness when you ride with snowboard. Just fly kite down and edge hard. Riding on snow is easier that riding in water.