one of our club members says
'copper is better electrical conductor then the steel in your car'
All other things being equal, perhaps. But, in a car, you have a lot more steel running in parallel to the battery than you would in a 10ga or an 8ga (or even larger) wire giving steel the advantage in the automotive application.
The problem with using the car chassis as your return is that you really don't know what path the current is taking and how vulnerable it might be. Is it passing through places that could rust out or lose their conductivity causing the the current to prefer other, less desirable paths, for example? With a wire, the variables are reduced considerably.
In my new car, the car's chassis offers less resistance than a 10-ga wire (probably larger wires as well, but I haven't tested that). 10 years from now (if I still have the car), it's hard to say if that will still be true. I'm thinking it would, given all the internal parts of a modern car that rely on a good ground, but it's a lot harder to know for sure than it would be with just a wire.