X pipe or H pipe, again

See - X pipe H pipe or side by side

In short:
H pipe - Lower end torque/power, deeper sound. The pulses will "run" into each other in the pipe. Helping reducing resonance. Think old school V8.

X pipe - High end gains. More horsepower overall. The pulses will "merge" better in the pipe. Causing a scavenging effect at higher RPMs. Think raspy newer high performance engines.

As you can see, some agree to disagree. But in the end. That's how it works out.