X-pipe will deliver a more high-pitched exhaust note, making a car sound more exotic, whereas an H-pipe gives a deeper tone with more of a traditional hot rod sound.
The X-pipe will usually make anywhere from two to four more horsepower than an H-pipe. The slight horsepower difference comes from the way the X-pipe creates a venturi effect, which helps pull the exhaust gases out of the cylinder faster.
H-pipe the exhaust has to make a sharp 90-degree turn, while the X-pipe offers a smoother transition. The X-pipe creates much more of a scavenging effect.