While I agree that the magnaflow muffler has the design for better performance... if all you are replacing is that muffler then you shouldn't care. The muffler is a supporting mod that compliments a ton of other mods that help increase performance. I think gains, if any, will not get noticed with just a muffler swap.
I personally chose the Flowmaster 40 and put on some 5" tips.
Ignore the slapping sound as that is my broken exhaust manifold bolts..