The X pipe vs H debate has alot of factors-
Most importantly which X pipe you are using and some are definitely made better then others-
I guess the best way I could put it is when I first finished my Shelby, it had an H pipe and Flowmasters on it- (yeah yeah yeah I know- they were on the car when I bought it)
After installing the Magnaflow X pipe and mufflers though- my RPM jumped up 150-200 RPM at idle because the car was breathing so much easier-
I forgot the exact numbers, but I picked up torque and hp across the entire power band-
X pipe is definitely quieter though then the H pipe- the sound is more NASCAR and less muscle car though- best way I could describe it-
On my 66, it is so quiet at idle, and so much louder above 4000RPM, I can usually scare people pretty good...lol
Working where I do, I could run an X or an H as we make both- after having both I will never run an H again though-
of course this is my personal preference-