The Nismo exhaust is more of a true dual design, where the non-nismo isn't. The picture above is actually on our shop 370z, which is a sport with the non-nismo muffler in that picture
Here is a comparison pic of the flow characteristics of the two. To be honest, I'm not 100% sure how accurate it may be. Aesthetically the Nismo muffler will come with much larger tips and the muffler
I actually have these two mufflers sitting right outside of my office, here's a quick cell phone pic