A lot of guys seem to go with the HKS on a supra. It will be louder by a good margin, not that either are quiet. I haven't heard a Blitz on an mkiv, but they sound good in pairs on a Z! I like the HKS myself as well.
I've owned both and I have to give the nod to the Blitz unit. You really just need to hear both of them if possible and decide. FWIW, the Blitz is more of a puuuhhhhhssss sound and the HKS seems to make a high pitched shreaky sound. Anyway, here's a pic of the Blitz welded onto my IC hardpipe.