lots would say H-pipe goes with a chambered muffler such as flowmasters and X-pipe goes with straight through mufflers like
borla or magnaflows. But everyone has their own opinion about sounds. I prefer traditional muscle car sound so i went with MRT stainless steel o/r H-pipe with magnaPACKS.. LOUD.
X-pipes will give you great higher end with better performance
h-pipes give you better lower end but less HP's.
Thing about MRT H-pipes is it gained the exact same amount of RWHP as Dr. Gas X-pipes did so you get the best of both worlds (sound and performance) that being another reason why i went with my MRT pipe.
All in all, i would say x-pipes sound more euro racy sounding if you will and H-pipes traditional muscle car sounding. Both are really great sounds. If you want you could try mustangexhaust.com or something and listen to samples. Hint: sound clips do not justify the real sound.. NEVER.. good luck
btw. yes, i'm korean.. heh