X pipe will make a little bit more hp than the h. Not much, but a little nonetheless. Typically the deciding factor with most people is just the sound preference.
Just go on Youtube and search different types of exhaust setups to get an idea of what you want. Also, you can try MustangExhaust.com - Find the Perfect Rumble! for sound clips. It will always sound a little different in person though, keep that in mind.