In the hope that my experience may be of help to others trying to select the right harness I thought I would add a few comments here.
Like many riders, I've migrated to kiting from other sports where the lower back takes a pounding. In my case, it's been a history of powered slalom water-skiing which works away at the fibrocartilege separating the discs in the lower back. Result being a weakened core and a lower pain threshold to sustain a powered stance against pull.
Harness choice therefore became key to me (more so than kite and board) and I've tried many options - Waist, seat and shorts all trialled. In 'my' experience the shorts just didn't work for me. Loved the concept but the load gave no spread of force across the affected area. To a lesser degree, the seat option gave me similar results as compression is still not fully supported. So I then went through an expensive waist harness trial experiment before I found one that suited my body shape and riding style at the time - both of which I believe were key to selection. Also, you should consider that as you progress (and age!), your riding posture will modify as your riding improves. I'm now at the point where my waist harness is not as firm as it was (wear and tear) I and have to move on or replace it.
The long and short of it is that there are so many variables to consider it's almost impossible to buy the "absolute" best for you at the time you want to buy.
My advice, given circumstances like my own, is to buy something that fits snugg without pinching when you consider yourself ride-fit; accept that your physique may change and the harness will soften quite quickly and replace regularly as materials and ergonomic science advances. For anyone with back pain issues, harness investment should be an absolute priortity.