im going to agree with Everyone else.
h= sound...also cheaper to find used(i have h)
x= power(wont feel difference between the 2 on a slightly modded car), more raspy little more money.
if i were to buy oneNEW i would buy a pypes x.
mine came with the h....so why change it

 

i've got a o/r x hooked to a mrt cat back and i find it to be very loud and i like it kinda. it sounds great at an idle and above 4 thousand rpm. but anything between and it is just way to raspy and obnoxious for me. i'm trying to get rid of the x so i can get a h. I like the the rumble of a v-8 but the x is just raspynoise at light to moderatedrivin and cruisin speed.

 

Just heard my pypes o/r x for the first time today, and LOVED IT. With the loudmouth 2s, it SCREAMS, and we only took it up to 3k, cant imagine what WOT sounds like.

Just a word of warning, the pypes x pipe was a BITCH to get lined up right, and it sits quite low under the car.