I love both vehicles, since I see them every night at work. But, my choice without a doubt, would be the G35. It's classier, and yet just as fast as the 350Z. (Factory, of course) The 350Z is for the single guy/girl with no children. The G35 is for the family guy, that still likes to go fast & look good.
The G35 has smoother lines all over the vehicle. The interior alone is awesome, especially with this:
But, I still love the 350Z, as well. For a sports car, it's simply awesome. Both cars were on the Edmund's top 10. The only 2 cars that made that list, from Nissan.