The x pipe is definitely louder because of the way it mixes the exhaust together. It is also said to have better flow since it goes into an "x" chamber. I'll tell you right now that Bassani makes a badass o/r x, but it IS NOT a TRUE X PIPE. If you want a true X, go with Dr. Gas, they make one hell of an X pipe, or so I've heard.
Again, an X is more of a louder ripping sound...here's a clip:
O/R X Pipe w/ Flowmasters