to me this question is very difficult... rather like what length harness lines do I use ?
the waist harness has come a long way, comfort features, and really good BACK support for anyone who has a lower back issues.
I almost don't remember the seat harness, have one somewhere.
I do think that racers are still using the waist alot, but for the vast most part almost all use the waist.
Try it on in a shop and feel it out. Lots of good... really good ones out there.
me ..on a Naish kiteboard one, best I have had in along line of them.