X vs. H
Many say that the X provides more HP, however, there have recently (don't have link) been some shops testing the difference on the dyno and showing that this isn't true. It may be true on some cars, but on the S197, the debate goes both ways.
That said, like many are saying the X will provide a much more raspy and aggressive sound. Also as many are saying, and H sounds MUCH better with chambered mufflers. I am running an O/R X with American Thunder AB's and it does sound good while punching it...but when you let off the throttle it sounds like apes being raped by godzilla...not clean at all...
If you run straight through mufflers, go X. If you run chambered mufflers, go H. HP difference really shouldn't even be considered IMHO.