Don't hold me to this. I'm not an expert, but I believe an x-pipe has more scavenging capabilities than an h-pipe. The h-pipe however has a deeper tone quality about it though. I've heard of guys doing both to get better power and sound. Have not verified though.
"No, it will never have enough power until I can spin the wheels at the end of the straightaway in high gear" - Mark Donohue
"You might think that's not cricket, and it's not, it's motor racing"
"Finishing second means you are the first person to lose"