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All depends what you want to do with the car. Big difference in what you are looking at going to car shows or tracking it. Modding it or leaving it stock. Daily driver or garage queen.
Id pick up a C5 coupe with 100k miles which is going for $11k-$12k where I live. Then make if far better than both the C5Z and C6. But just because thats what I did doesnt mean its for everyone though..
C5 Z is a good bang for the buck, but MAY not be enough power for you (wasn't for me) If not, you would STILL need to mod and there goes your MORE $$.
If I had it to do over, I would do exactly what I did the 1st time, bought a low mileage C5 and modded it MY way. Now it looks as good and OUT-performs C5 and C6Zs
Since I have to have the option of an open top, would go with C5 coupe or roadster and build around that for more power. Like the other poster said, you won't get much of a C6 for the low $20Ks.
I am guessing about $5K difference between them for nice ones. (C5= $15k, C5Z= $20k, C6= $25k)
For that difference, if you are not sure which you want, I would go with the nice C5 because of price. Try it and drive it for a while. It is a fun car to drive and at the current prices is a lot of car for the money. I think it is the best bang for the buck.
You may find that it is the way you want it or you may want to improve performance (easy mods for acceleration or handling).
Later if you do decide that you want the C5Z or the C6, then you can still sell the C5 without losing much since they have already dropped pretty far and the C5Z and the C6 may still depreciate more (especially the C6 since the C7 just came out).
All three are great cars, so you can't really go wrong. You should drive some of each to see what you like best.