I go with the Porsche. You already have a 540, so no need for the M5.
z4 is too small. Porsche has a little more room and faster.
as for the viper, my neighbor down the street has one, he says its handles like crap. BUT its fast...so if that floats your boat.
Porsche seems to be a good combination of a weekend car and drive to work car with enough balls, show, and practicallity(if thats even a word)
We all get the urge...Saturday early morning...nothing to do till noon...
The aural sensations of 1st gear WOT
**CGM **(Certified Garage Mechanic)