which 3'' exhaust do i want?

You do have to make a lot of hp (500+) to warrant a 3" system. Most exhaust systems are stepped down throughout the system in order to keep the exhaust gases hot and moving. Cool exhaust gas will not flow as well and without some back pressure will affect your low end torque. If you`re running open headers, you`re probably racing and should tune your engine accordingly. With open headers the idea is to get rid of the exhaust gas as fast as you can because you`re running nothing but wide open throttle. In a street application you`ll want to maintain a high exhaust gas temperature so the system will flow. If you make a lot of hp and need the 3" system ,then go for it, but try to maintain high exhaust temperature by stepping down the pipe at the muffler to a 2.5 or use another method to create back pressure and thus higher exhaust gas temperatures.