As Jay said, 370z from 350z anytime, but Nismo 370z is the ugliest Z ever produced by Nissan with a price difference from the standard version unjustifiable. You basically pay a lot more for some red and silver badge, crappy exhaust, a stabiliser bar and stiffer springs, all under an extremely ugly body kit that is ruining the beautiful look of the 370z. £6000 extra for that?! You can get a supercharger fitted for that kind of money, we're talking about 500rwhp and 400lb/ft torque.
The only things worth on the Nismo version are the wheels, a lot nicer than the standard ones on the sport version.
Performance wise, it's a joke, 16 little hungry ponies extra...
I'd get a beautiful 370z GT and start playing with it, better exhaust and suspension, UpRev will give you the extra power difference and so on.