OK. Before you make a bad decision, I want you to understand something about a C5 with lowered suspension. It might not be as big a deal as you might think. All C5 Corvettes have adjustable suspension with about an inch's range. All it takes is about a 30 minutes time of your corvette mechanic and another 30 minutes of alignment to get it back to stock. Now, some people take that to another level and cut or remove bushings and such. This is NOT a good idea. Another way to lower the C5 would be to purchase a coilover system. Those systems are VERY expensive and if it's on the car, I might mean that the person chose not to spare expenses.
Having said that, by all means, if you have the ability to purchase a Z06 over a regular C5, don't hesitate. They're wonderful cars and few can point out a flaw in them.