There are minor differences in power at different RPM levels, but its probably not enough that most folks would notice.
In most cases, the exhaust note "pitch" with an H-pipe is lower and has a rumble, while an X-pipe is higher pitch and more raspy. I think the H-pipe sound is closer to a classic muscle car sound, while an X-pipe is more like a newer high rpm, small displacement engine (tuned Mustang or Camaro).
The other consideration is that an X-pipe is hard to fit on a 4x4 unless you want to lose clearance. There just isn't a very good place to fit one around the long transmission/transfer case and driveshaft.
Bruce