Get the mandrel bends. I'm not sure about sound, but they definately have an effect on flow. Think of it this way....when you've got stock exhaust, one of the reasons it's restrictive, aside from the cats and cheap mufflers, are the non mandrel bends. If every aftermarket exhaust company out there mandrel bends their pipes, there's gotta be good reason for it.
mandrel bend ensures true flow through the entire exhaust, rather than having various bottlenecks at different points in the exhuast, if you do a custom exhaust u usually do a mock setup using a regular pipe bender than send the mock up out to get mandrel bent.
take stock headers on each primary there's about 2-3 bottlenecks so thats why aftermarket headers make a difference in sound and power on a car.