Powerflow are fully customizable as they are built for you while youre waiting. However, with the Magnex you get system that has proven results. You may get more or you may get less from the Powerflow system but you wont know until you get it dynod.
I have a Powerflow system (2.5" pipe, cat back, no central resonator, tiny back box) and can only praise there workmanship. Lifetime guarantee also.