Not much difference between the two performance wise. The h-pipe will give the classic muscle car noise, the x-pipe will give it a more modern v8 sound. It's mostly about what will fit the car and you budget, plus what sound you prefer.
Ever consider a dual in/dual out muffler? I had one and the sound was amazing. Really smooth yet muscly sound. I really liked it. I don't know how it would affect performance vs a dual muffler system though. I do believe that some sort of crossover, whether it be h or x pipe, or a dual in/out muffler will yeil better performance than a dual unbalanced system.