Originally Posted by
Petrol & Powder
As for that last line, "Copper also is a better conductor of heat."
I'm not sure what the OP means by "conductor of heat", but I'm positive that when talking about a gas check, it's a non-issue.
The length of time heat is applied to the bullet base is extremely short. And while both materials will obviously conduct heat, the copper is denser than the aluminum (again, a total non-issue in that application).
So of the factors the OP lists, I think the last one is a total non-issue in that particular application.
Expense, ease of forming and maybe weight would be far more critical factors.