Don't know which ones you're considering but here's a picture that some good citizen posted some time ago that may be helpful.
I've seen dynamometer data showing a small horsepower increase with the Aries muffler.
I've been pleased with my Aries muffler. I decided against the stainless version because the metallic shine just looked too out of place, even by my liberal standards. One of the first things you might appreciate is how well the Aries flange mates with the exhaust manifold--assuming that the latter has not been too warped and sagged by overheating.
Steve