from what i understand is that the preflex repostions your shock straight up and down to give it a smoother ride and in the process of relocating you shock mount it just happens to lift your bike about a inche and to answer which is better it depends on what you want the preflex will give you a better ride but not as much of a lift as a HL lift kit would but with the HL lift kit it gives you a rougher ride so basiclly
Preflex - one inche lift - softer ride
HL lift - two inches - rougher ride
Preflex - one inche lift - softer ride
HL lift - two inches - rougher ride