I think they both sound good. I've heard a lot of good things about Porters as well. Like said before, where the mufflers are mounted (close or far from headers) and the diameter of the pipe you choose to run can change the sound quite a bit.
Those Smithy/Porter style mufflers set up right sure do produce a nice hot rod six sound, I really like it.
In my neck of the woods most cars/trucks with inline 6's I see at gatherings/shows/driving down the road are usually running straight pipes off of either a stock split manifold or Fentons with 1 1/2 or 1 3/4 inch pipes. Very loud and a lot of rap, but that is what those guys are shooting for.
