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X pipes are shaped like an X, their design promotes better flow of exhaust and therefore a slight increase in horsepower. H pipes are H shaped, hence the name. I prefer H pipes because the sound is more "american muscle" while X pipes have more of an exotic/import type sound.
I have a Bassani X and my car sounds nothing like an import,and if you are worried about sound more than power might as well go rice with a 3ft high wing well your at it.