1. Primarily Sound/Performance.. The only exhaust i read about that had dyno'd evidence of improvement in performance was the Tubi.
2. Looks. How the tailpipes looked from behind.
3. Quality. The build quality, the fit quality, longevity.. etc
And price was an issue but they all seemed to work out more or less the same money wherever you bought them from and however you built them up. Beastpower were the best price for a cat-back system that i could find.
its really ashame that more peole do not buy the B&B's as they are not only the lightest setup out there, but also they create the most torque out of any setup that is offered. Bav Auto has a dyno sheet from a run and it shows an increase in torque of about +10 across the band and about +6 in power.