How much are you willing to spend? You could start at a mac o/r h pipe at around 130 bucks, or go for a Bassani X pipe for several hundred. Check out mustangexhaust.com for some sounds which may answer the question better than any of us can.
Basically, an X pipe will allow for more power, and will generally give you a raspy nascarish type sound. An H pipe will give you the old classic muscle car deep sound, but won't net you the gains an X pipe will.