off road if your state doesnt require emissions, but if they do get the hi-flow catted ones ...they perform almost as well and aren't too much quieter ...If you are going off road, www.mustangtuning.com has good ones ...they carry the SLP catted ones too
If you are going for sound ...X pipe gives you a smoother, racier sound and the H gives you a more burbling sound. Its all a matter of opinion, but I prefer the X pipe for both tone and performance. X pipe with flowmasters: has a clean sound and really screams when you get on it
Alright. I cannot speak from experience here, but from what I have read on here. Alot of people like a Dr. Gas X-Pipe with Flows. There is no sound clips out there that I am aware of except one. But this is supposed to give you the best dyno proven gains....actually type a search in and you will be able to read ALOT of threads about this subject
An O/R H-pipe is the only way you should go. This is my opinion thou. I feel this gives the best sound for a mustang. Definetly if you go with some cams later on down the road.