<off topic> So the Mustangs transfer sound into the cockpit to give the driver a more connected feeling? That is kinda crazy.</off topic>
I went with the Berk Cat Back (GB#2). I like the sound of the F.I. Gemini, but I plan on lowering and that unit looks like it will have trouble with bumps, pothole, <insert random illness here>. The Stillen and Nismo units look to be similar to me, using the stock location and what looks to be stock clearance. No info on the other though. But there is a good thread that has all of the sound clips / videos in it.
370Z Exhaust Sound Clip data base Hope this helps you.
P.S. You gotta admit, some of that stuff above was funny. I just didn't expect to see it here.