I've always used a seat harness as you can really sit in it. I've never tried a waist but once you get the hang of the seat you can hook in/out on demand.
Maybe a waist harness would be better for B&J... easier for hooked in landings maybe... I don't know but definitely tuning of your rig/harness lines/mast track/etc will make a much bigger difference on your overall performance than whether or not your harness is seat or waist.
You see guys like Micah Buzianis using waist harnesses in the PWA Slalom as well so that sort of shows you how much crossover you get between the two types. Best bet is get your kit tuned right then if you can try both types out before buying.
JP Super-X 106l, North F8 Ram: 5.8m,