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It depends on what you like sound wise. An H pipe is a deeper tone, X pipe raspier. The mustang comes stock with an H pipe, But an aftermarket one is a bigger diameter and eliminates the two cats up near the manifolds (if the fox bodies have them...). I would look around you tube an listen to some sound clips to get a better idea of what you like. Fwiw, My new edge is running a borla stinger cat back with a magnaflow catted X pipe.
Just as the nice gentleman above me said, H pipe gives it a deeper "dronish" sound, and the X pipe has that raspy snap sound to it. All about what you prefer. I'm pretty sure the horsepower benefits are similar. The idea behind the H and X pipe design is to equal out exhaust pressure on both sides and allow even flow and better exhaust gas evacuation. Common sense would say the X would tend to flow better due to its design having less extreme angles in it though.
Put a set of BBk long tubes on it with a BBk shorty off-road Hipe and a flower master catback with some Super 44 on it. Whats on mine and it sounds bad ***. Chevy guys love it!