I've had a few combos on each of our stangs...O/R H, catted H, and now O/R X's on both.
IMO, O/R H or O/R X is simply louder than catted of course, but I'd be hard pressed to consider one louder than the other (H or X). They are different, but, depending on your combo, you could end up louder with either an H or an X.
My wife's O/R H with Flows was pretty outrageous, but I think my Borla Stinger catback with an O/R X is a louder when romped on...yet, if you compare idle, her's was nastier (kind of the nature of an H - it's just not as free flowing).
That brings to mind another myth - long tubes are REALLY loud...NOT true. Typically, and again, this is just my opinion but it is based on my own personal observations, not what I've read somewhere...long tubes (dunno have any experience with shorties, but I'd bet they would behave in a somewhat similiar fashion) quiet the exhaust down.