The thing that swayed me is american designs on their V6/8 engines and most use true dual of some sort (x/h-pipe or no cross over) no offence to the jap die hard fans (myself included) but they have more experience of straight or inline engines = single pipe back.
Depends on what you want top end hp = free flowing with a small back pressure or if you want mid range torque add a bit more back pressure = my problem at the moment (too many tight bends near the rear).