Buy both. They both have ther pros and cons.
I'm a 90kg rider so need to be holding down a fair amount of kite. I find that when the wind is light and the kite has to be worked and/or kept high in the window to catch the wind, the seat is far more enjoyable than the waist.
I really enjoy the waist when I can go on a small kite in stong winds - i.e. 7m in 25kn+ as much of the time the pull is from the kite low in the wind window.
A harness is one of the most important bits of kite kit you will buy and very good value considering the years that you get out of them. I have just worn out my Dakine Fusion seat after 5 seasons of punishment

, kept the spreader bar and paid $100 for a brand new Mystic seat without the spreader.