It is just amazing how the internet creates all kinds of engineers.
One click and you have the right answers and formulas...
WRONG!
I like to visit my last statement:
"The aluminum cool down easier and they don't need to be very thick. All what a heat shield does is keep the heat directly from the surface."
Ergo, alu heats up faster and the heat conductivity is higher. With other words, if there is any little airflow striking the alu the radiation will be lower, because of the rapid change of molecules (principle of heat sink).
Both shields will heat up to the same temperature inside the engine compartment eventually (no airflow) - the alu just a little faster, but is cooled down and radiates less on the other side when exposed to airflow.
As soon the fan kicks, in you have airflow.
As I said before, there is no gain (SS) with higher cost, more weight and harder handling.
Google tip:
https://www.google.com/search?q=automotive+heat+shields&hl=en&client=fire fox-a&hs=sbb&rls=org.mozilla:en-US
fficial&prmd=imvns&source=lnms&tbm=isch&sa=X&e i=0P5hUM2EIoje9ASNkIDACA&ved=0CAoQ_AUoAQ&biw=1785& bih=913