Harnesses are like snowboarding boots, i.e. each manufacter will make different sizes, shapes and designs. You need to find the one that fits your unique body shape.
You also need to think about your skill level and what kind of kiteboarding you plan to do.
* beginners should go for a harness that doesn't budge, not super tight, but tight enough so when you are chucked etc your harness is in the same place you left it and therefore easier to locate. Look towards the seat harness and remember to get the padded straps for between the legs so when you are doing the pooman it doesn't hurt so much.

* integrated shorts/harnesses are not my thing, but if you want it "all-in-one" then try out the extra-large nappy.
* waist harnesses are the most difficult to fit into. Each manu makes them slightly different, but generally speaking if you have a swimmers style body(i.e. wide shoulders, scrawny waist and stick legs) then the waist will fit you. If you are a bit tubby or full of beers in the belly then the waist will certainly rise up.
The mystic warrior i own has been perfect for me. I like the way it rotates for toeside etc, this will cause a rubbing effect, but you do toughen up, or some vaseline/sunshirt/wetsuit top under the harness will reduce wear and tear.
Find a shop with a lot of stock, wear whatever you would normally kiting into that shop and try them out. Nothing more important, IMO, than a comfy harness. I'd recommend(especially newbies) to spend extra here initially, you won't regret it.