I used to sell harnesses, 5000 annually, mostly waist, why? It's mainly a vanity thing, everyone thinks waists look cooler, but seats are more comfortable. Personally I use the DaKine Nitrous shorts but with a variety of bars depending on what I'm doing, wind or kite surfing, leisure or racing. Unless the measurement around your chest in the armpit area is bigger than the measurement around your waist, don't even consider waist, they will ride up. These shell style harnesses currently the rage? Mostly Bullshit.
The original shell harness, made by the original Ride Engine company was a custom fit, actually made to measure these latest types professing to use only three size scu's can't possibly do the job for everyone. Prior to them probably the best harness company, Dakine, made 5 sizes and used three spreader bar widths. Some might use a small harness with a 12 inch bar, or a medium harness with a 10 inch, there were lots of combinations that would assist fit. The current trend of trying to fit all body shapes in only three size offerings is BS, so you have to try around, as many harness types and brands as you can, to find a decent fit. We are all different.
The other thing that is important with the two different genres, waist and seat is hook height, with a seat harness the centre of effort is generally lower than the centre of gravity so you might find controlling your position in the air a tad more difficult than with a waist harness which has a higher hook position above your centre of gravity which means the area below the hook is heavier than that above so airborne manoeuvres and tricks are easier to control and land. But, you can have seat harnesses like the Nitrous shorts with a high hook and they don't look to bad in warm water locations just riding in them and a rash. Don't be afraid to experiment, Other than your kite and board, the harness is the most important buying decision you'll make.