I wouldn't. I doubt many people can afford the price of the Z but not the price of the Mustang... so it seems to me most of us already made the decision.
I would have a problem paying that much for an American car -worrying about how well it will hold up and what the resale will be like.
Also, I think the 370Z is a much nicer looking car than the Mustang.
So, last week I had to make the choice and I chose the Z