I believe the rule of thumb is H for chambered mufflers and X for straight-thru mufflers...that being said, yea basically go by what sounds best to you.
To me X pipe and straight thru gives you a more exotic tone (esp on a 4v) and all then hell breaks loose when you go WOT while the H pipe chambered gives you a mean old school low end muscular rumble...but no matter what you have you can still always tell if it's a Stang or not.
As it is I almost scare myself sometimes if I'm flooring it through an underpass or tunnel.