Sorry too many replies to read but I want to give input because you are in the same situation I was in one year ago.
I LOVE the 350Z because it is a V6 with the same HP and a bit less tq than the 03 GT I ended up buying. Now, here is the situation.
I looked at:
Gas
insurance
comfort
performance
Gas is nearly the same in both cars, maybe slightly better in the 350Z, I have to say, you will learn how to drive a V8 and make it efficient when you need it to be!
Insurance my GT is less on insurance than an 03 350Z would be for me. Maybe a couple of bucks less, but you take those extra couple of bucks and it equates to= beer, going out to eat, mods for you car, going to a movie...etc, etc, etc.
Comfort- I am 6'4" and although I can drive a 350Z, I am not at a daily driver/road trip comfort in the car.
Performance, the 350z offers a completely different level compared to the Mustang. My other criteria outweight the 350z so I favored the Mustang, PLUS the price was WAY less. I found several 350z's for $12K that had over 100K miles. I found several 50-80K mustang GT's for $12K and then I found MY GT with 35K miles, fully loaded(exc. mach 1000 audio). I had to drive 1.5 hours to pick her up but that was a small price to pay.
Now you want to compare the cars for performance, I personally feel my Mustang is faster. In a straight line that is. The 350z out handles and can out drift the Mustang anyday. But when your on the highway or at a red light. It won't matter how much better he can drift or pull G's. | 
