For learners who a lot of time fly their kites up high, a seat harness is more forgiving as it prevents riding up. Seat harnesses have a lower hook and with the leg straps one can anchor the weight down a bit easier. Waist harnesses have a higher hook one can come in and out of it much easier when doing unhook tricks and for other reasons kusu has pointed out correctly.
The more mature riders who doesnt unhook as much, also those with back problems tend to favour seat harnesses.
Liquid Force has mens and ladies boardshorts harnesses with leg straps inbuilt and detachable bar so you can wear it as a pair of boardies say when you are heading the beach or going home. 08 boardshorts are on clearance at all LF distributors as 09 stock due in the next shipment on the 28th this month.
Care not what others think, care what works for you best.