I think both are worth their weight in a heaping pile of cash.
![Big Grin](images/smilies/biggrin.gif)
It would come down to whether you are comfortable buying a used sports car. I would heavily recommend against this, as you don't know its record. You don't want a car that has been beaten to hell; even if it has a "warranty". Speaking from experience, used car warranties are not as reliable and forgiving as new car warranties. And as someone who previously dealt with a used sports car that was billed as "certified pre-owned" and came with what I thought was a helluva warranty, it was a mistake. The car had more issues than it should have; due to the previous owner modifying things and the dealer not knowing or caring enough to put things back to stock order.
After all I went through with a used sports car, I can't suggest you get the used C6. The brand new Z is a better investment in this scenario.