I have an old Super Ford magazine article where they dyno compare the two, and the X makes something like 6hp and a few tq more than the H. It's so little that you'll never know the difference so go with what you like the sound of most unless you're building a true racecar and you need every single hp you can find.
+1 on the 'X'. Im running shelby Tri-Y longtubes, 2.5" to an offroad X and SLP LM II's. People have told me it kind of loses the "classic ford pushrod sound" achieved with dumped flows with an H, but they compliment it none the less as a crisp, refined sounding exhaust, almost mod motor like?
video:
YouTube - Shenanigans
Sound clips at 0:07, 0:30, 1:02, some fun at 1:35. Ignore the rest of the video, including vehicles involved.