If your radiator is outside your rig and you have waterblocks on the major heat producing components you don't need more than 1 or 2 fans for your case - getting alu just for lower temps definitely not needed
Of course if rad is inside case than different story - however, have a read here:
http://forums.procooling.com/vbb/showthread.php?t=4863
If you setup the loop properly, I'd say alu vs steel will have negligible difference - of greater importance is airflow inside the case. If I were to guess I'd say maybe 1 degree difference at most if rads are near the openings with plenty of airflow.