Invidia is deeper than stock, I actually prefer the tone. For some weird reason, when I hear G's and Z's, the stock exhaust sounds like it doesn't suit the car, almost too high pitched. Probably just me though since most people love the sound. Mine is customized w/ a Magnaflow resonated X-pipe to reduce some of the volume and rasp. The resonators that come with the Invidia are small and don't do much. So if you get the Invidia and it's too loud, just replace the stock resonators with a resonated X-pipe. It's only $100 and it's the same thing that the Z and G guys use on their eBay exhausts to cut rasp. For the most part, the Invidia is unnoticable, but there is a drone at 2-3k rpm. It's there on the stock exhaust too, just magnified with the Invidia.
FWIW, if you like the Nismo but find it too expensive, you can pick up a Fujitsubo. Basically the same thing, and they go for pretty good prices used on the G and Z forums. It's a Y-pipe back, so you can use your stock Y or get an HR one or aftermarket Megan or Topspeed made for the HR, those will clear the transfer case.