If the engine is stock, and you plan on keeping it that way, I don't recommend purchasing the complete exhaust system - look at the Thrust Chamber instead, it will deliver better power for your stock engine than any aftermarket exhaust system.
If the bike has (or will have) the 150cc kit and a camshaft, the Powroll pipe was designed to go with these. It was designed at the same time we designed the bore kit, camshaft and everthing else.
Basically, the Powroll pipe is geared more towards overall power, the BBR pipe is desgned for m/x and for more top end use. For aggressive riders who race, the BBR pipe works well, for the rest of 'em, the Powroll pipe would be the best bet.
BTW - I work for Powroll - so if anyone has a more "unbiased" opinion - please throw it out here!!!