I don't think it's black & white- you'll find people who'll swear by each of 'em.
Meself i'd go for the X just cos it looks more efficient.
I made one for me Truk (Rover 3.5 lump), hard to give a comparison tho as i fitted K&Ns and rebuilt the carbs at the same time
From what i have read the X pipe will work better, H pipe is what i have, think we are talking minimal HP increases
But the X pipe will look cleaner and be easier to fit up
ALL KILLER NO FILLER
Nostalgia, its not what it used to be.
I believe the X pipe gives the better flow. I have read that quite a lot of guys are not happy with the way an X pipe alters the exhaust note however, they say it doesn't sound quite like a V8 any more. I don't have any personal experience of this, but it usually gets mentioned on message boards when X pipes are being discussed.
Went for X pipe on the Cuda, the main reason being to try & quieten it down some, as wanted to try & get away with a pair of exhaust boxes as opposed to 4.
Hopefully this will work as getting tight for room underneath.