One would think that if you spend that kind of cash on a track oriented car such as the zo6 you wouldn't have spend additional $$$ to make sure it doesn't blow up and cost you 13g's for a new engine. That right there would turn me towards a c5 zo6 just for piece of mind.
Danny Popp even said the c5 is a much better track car than the C6ZO6, The article was in a previous Grassroots Motorsports in which they asked what was the best open track car.