stainless steel vs aluminized steel, mandrel bent vs regular bent

I guess the main question is how much performance is really affected on a car like mine.

Its not a huge HP engine...its a 318 that may be tweeked a little.

The local shop I am considering is an easy fix...they will flat bed the car down cheap, and the guy does seem to know what he is doing as far as I can tell...I have just always heard that mandrel bent is best..I have a mandrel bent cat back on my truck, but cant honestly say I would know the diff between it and standard bending....