walking up wind with a seat harness enough will give chaffing-its great for beginners, makes you want to learn to go up wind real quick other than that its up to you,----fat men shouldn't wear waist harnesses LOL
I started kiting with a Dakine Fusion seat harness mainly for the fact that its a very comfortable harness.
But i ended up switching to a Mystic Warrior waist. generaly i would say as a beginner a seat harness is easyer and less painfull to use but the Mystic Warrior is extreemly comfortable, and have to say its easely as comfortable as the Fusion.
It pretty much comes down to personal choice. just try em and make up youre own mind. I switched mainly because i wanted more movement in my hips to get more into wave riding and give a bit more mobility.
It depends a bit on your body type as well. If you have the inverted triangle look going on then a waist harness will usually be fine. If you're a bit thin then you might have trouble with a waist harness. I've tried both and found that all too often the waist harness ended up being an armpit harness - not cool. Consequently, I ride a seat. 
Been discussed before but in short, unless you get a well fitted seat harness, do your crown jewels a favour and get a waist(north stylers and the mystic ones are meant to be alright).
Matt
If you want my opinion, I'd go for the waist harness. I only started kiting at the beginning of tthe year and the shop said that the seat was the go to begin with but I have since traded the nappy for a waist and it was the best thing I've done. When you start up, you will be doing heaps of walks of shame and it is no fun with a nappy on cutting off the circulation to your legs and pushing your nuts into your guts. 
. The waist rides up a little and you'll be a bit sore for a while but I've got used to it now after only 2 weeks of using it
