Depending on how you are going to use the car, IE cruising, car shows, or having fun and racing it, here is the question I would ask first.
What happens when something ZR-1 specific breaks? Where do you get the replacement parts? Not too many options.
What happens when something Turbo Buick specific breaks? Tons more options. Very rarely do you hear about a GNX specific part breaking. IE: rear end component, etc etc. Besides, most GNX owners who like to run their cars, take the stock ceramic turbine wheel equipped turbo off and replace with a standard steel wheel equipped turbo anyway. I would take the GNX, you would get tired of the ZR-1 a lot quicker than you would with the Buick. And the Buick is a lot easier to work on as well. Lotta upgrade parts too.
Just my O2's worth.
Patrick