I'm not a big fan of any of these, but Borla seems to be very popular and I hear the Nismo one might be a little overpriced (not sure though). And, if I'm not mistaken, there are two Stillen exhausts. Which one were you looking at?
I assume you mean street legal. I believe the first GReddy (was it the SP2?) is the only that has come out for the Z that isn't. Since this stuff is handled state to state, I suppose there is a chance they aren't all street legal where ever you live, but you shouldn't have any problems with those three
If you check out the classifieds on this site, you'll see Chazz has an exhaust system for sale. All 3 you listed are good, but Borla has the most pieces, and I think it's also the heaviest (not sure though).
I would also check out the modifications sections and see all the threads about exhaust. Track just got a Greddy, and I've personally seen and heard the Injen SES (which is nice).
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