Riding style, and what you ride. The trail/harescramble racer isn't going to like the same pipe as the hillshooter, duner, drag racer. MX racers don't run drag pipes, and vice versa. Jerrys, RK's, either of the Matts pipes are going to have their own specific powerband that better suits any number of different uses. How does this ALWAYS end up turning into this?

Hateraid, seat of pants dyno, real world... Whatever.

Dune riders ask about dune pipes, hillshooters ask fellow hillshooters, XC guys ask fellow XC guys. That seems to make too much sense to me I guess.