How much butt do you need to haul? If more than 2, BMW, 2 or less, Z06.
The BMW is a cool car with great build quality and driving dynamics but you will *definately* feel the extra weight in the corners. I've driven a couple last-gen M5's and while very nice cars they're pretty hefty at 4025 lbs. The Z06 will be far, far more "tossable" with 1000lbs less to haul around.
The BMW is an amazing technological tour-de-force, however. Depending on your mindset it's either really cool or has a LOT of stuff to go wrong.
For all the cool technology, the BMW has pretty bad fuel economy estimates (10 city, 21 hwy); I bet the Z06 will pull about 20 city / 30 hwy. For some reason it seems nearly all hot rod german cars have pretty mediocore fuel mileage. Not that mileage should be a determining factor when you're buying a car like this, but I've never understood why their mileage is so terrible given all the cutting-edge technology they employ.
As far as what I'd buy: Z06. I'm single and don't have to worry about hauling a family around!