Actually the OEM and OE replacement stuff is aluminum. I don't know that even the aftermarket stuff uses any thing but aluminum.
The only rub with the ACDelco replacement units is that they are universal with respect to transmission. They all have the cooler in the right tank for the auto equipped C4's.
The aluminum is not an issue as long as you keep up with coolant maintenance, keep the grounds clean, and have no gasket leaks.
Tom