Of the ones I have seen, and I used to fabricate them, Dr. gas has the best designed. This is because the X is as close to the header as possible for a mass produced pipe. If you find an artical on X's or h pipes you will find that they are generally close to the header, as where Bassani and others are right by the flanges.
I currently have a catted bassani X (because with cats X placement is a mute point, and it was cheap too me), but when I buy an off-road pipe I will buy a Dr. gas because of the good design.
Good luck.