H-Pipe: Uses the Hemholtz resonator theory. The H pipe's ratio to the exhaust pipe will cancel out a particular frequency, when done properly it will reduce and in some cases, eliminate drone. The sound from an exhaust is more refined using the H pipe as you have each banks' pulses travelling for the most part independently from each other. On a V8, this gives you 4 pulses in each side. Sounds good.
X-Pipe: There are variations of the X pipe that aren't true X pipes, there are kiss pipes and merge pipes that look like an X but aren't. A true X pipe will be a definite X that does not change pipe size throughout the whole X. Magna Flow make one. A Kiss pipe is basically two bends that touch with hole cut out where they meet. This is a "cheat" version of the H, better than a merge but not really effective in tuning out frequencies. It will have a small effect. The other type is a merge, these are bad for flow as they are similar to a kiss but have a much larger contact area, this reduces the diameter of the exhaust where they meet. Very restrictive. Now the true X is the best of all 3 as it does not reduce in diameter. The effectiveness of an X depends entirely on where you put it, the theory of an X pipe is to create a low pressure area behind the exhaust pulse when it crosses over into the opposite side and this aids in speeding up the pulse behind it and results in a scavenging effect. Positioning is vital depending on what the desired outcome is. Too close to the engine in high performance applications and it will actually reduce flow as the revs get up (high revs result in pulses that are close together and will cause pulses to crash into each other at the X) Further back and it will act as it should. In street driven or low to mid requirements (IE track work) too far back and it will pretty much do the same as the exhaust pulses are slower and further apart, they will also cool faster and "bank up" at the X. Sound wise, all X type merges tend to muddle the note as they combine all 8 pulses into each pipe because they are open to the flow from each header pipe.