My opinion would be to let your father decide what he wants. I always hear the C5/C6 debates, I own and simply love both, for completely different reasons. In reality they are both Corvettes and always will be. I loved my C4! I think the C5 is one of the greatest cars ever built! The 02 Z06, another all time great car! The 09 ZR1, once again, Chevy has hit the mark and has continued to improve, not only performance, but ride, quality, reliability, etc., etc., etc.
I love my Z06 and will never get rid of it, it is light, fast, loud and handles like it’s on rails. It also gets fantastic gas mileage for a 405HP automobile, but I can call a spade a spade, the shifter sucks, the road noise is horrendous on long freeway trips, etc., but I love that car and I love driving the car. Every time I get in it, It still feels like the first time, and I am glad to see a bunch of guys just as passionate about the C5.
The ZR1 is the first C6 I have owned, but I absolutely love this car too! It does not the raw racer feeling of the C5, but it is unbelievably fast and handles like nothing you can imagine, but it is also quiet, shifts like butter and is a lot more comfortable than the Z06.
So you can argue about which is best, but you know what guys? They are all great! Good luck on your dad’s decision.