X pipes and H pipes are meant primarily for sound benefits, with each having different characteristics. H pipe will give little to no performance gains, and the X pipe will help improve flow a little, but probably not enough to be noticed in a street engine. The X pipe would also need to be a custom setup to see the best gain, because the distance it needs to be away from the engine will vary with every build. The over-the-counter setups are basically an approximation of the best location. Unless you're building a high horsepower street/strip engine, your best bet is to go with the H pipe. X pipes can also be a royal PITA when working around the transmission and drive shaft.
