It's mostly an issue of personal taste in sound/tone. Power-wise, I've read the X-pipe allows for more horsepower, which an H-pipe is better for torque ... but in either case, the difference was less than 5 HP or 5 ft. lbs. on a built-up application, so it's highly doubtful you'd feel a difference between the two at all.
As far as sound goes, with an X-pipe, think of NASCAR; with an H-pipe, think of either the Mustang or the Charger in "Bullitt."
FWIW, I've found that chambered mufflers seem to sound better behind an H-pipe and straight-through mufflers sound better behind an X-pipe; chambered muffs behind an X-pipe sound kinda weird, all hollow and gargly like a Camaro or a GM V-6, while straight-through muffs behind an H-pipe sound too pickup truck-like, sorta like glasspacks.
I would go H pipe for classic sound. Not sure if the X sounds different at idle, but I would easily give up a few horsepower for the muscle car idle sound!
ive had 3 x-pipes on my car BBK, Bassani(current), and flowtech.
the flowtech had the smoothest sound by far, if you look at the design of the flowtech one, its X section is right under the trans, and i dont know what it was but those pipes sounded so good... the only downside was the cheap flanges, but i think it was only like 120 bucks back in the day
I thought I read before that the x-pipe is better in terms of performance... But I can't verify that now. I know that I had a reason for putting an x-pipe on my car, and knowing me, its because it was supposed to perform better.
Anyway, my vote is for the X. The sound is definitely more race-car like than the H.