[Aussie Mike]

A V8 is only firing one cylinder at a time and usually on alternate banks (depending on the firing order). That means on a true twin system that there's a fair bit of dead time in the exhaust system between pulses. By joining both sides together with some kind of cross over you are effectively doubling the flow of your exhaust. Each pulse can travel down both sides of the system.

From what I've read the X pipe flows better than a H pipe. The gasses aren't having to do a right angle turn to travel up the other side of the system. Just like a properly designed collector the X pipe can actually have a scavenging effect on the other side of the exhaust system.

The H pipe is easier to install as the two sides of the system don't have to converge together. Each has their pluses and minuses.

Hope this helps.

Cheers