I had a G35 Coupe for 3 years and was very happy with it, very well built car, my dad liked it, so I sold it to him.
I've owned the 370Z for 9 days now, and I can tell you one thing, fun, fast, for the win Z. ;-) Who needs all those people tagging along anyway, one of them is bound to spoil the fun. Grab a good friend or a girlfriend and go for it.
I think it boils down to weather or not you really need to carry 4 passengers. If you need to drive clients or customers along for work or if you have kids, I'd get the G37 sedan. If not get the Z.
One final thing, get a good test drive on both cars, you will know then.
Braden