the dual system will give you the best of both worlds. your pipes will be small enough to keep velocity/scavenging up at lower RPM for better torque, but by having 2 pipes, you can still get high RPM flow out of them.

2 2.5" pipes have a lot more cross sectional area and flow potential than a single 3" pipe.


but, for a truck (assuming that's what this is on since your asking about duals vs single) it can sometimes be tricky routing a true dual system, especially if it's a newer truck where the tank hangs inside the frame rail, in front of the rear axle, forcing your driver side exhaust to cross over to the passenger side.

how "big" is your engine? a single 3" system will still support a TON of Hp. my engine is only lightly modded, and put down 300 hp to the wheels, I do not feel like my exhaust is the limiting factor, or anywhere near choking me down yet. my exhaust will flow more than my heads will