The answer I got when I asked a friend who is a custom exhaust builder, before I had my dual stainless built was that there is a slight gain in HP and torque due to the better flow and less back pressure, both will somewhat lower the tone of the exhaust. The "H" is what he recommended for street and the odd strip run. The "X" is better used in high rev application, ie highway driving with 4:10 gears.
Hope this helps.