Strictly looking at OEM setups available to us, the iron caliper is better. Insert aftermarket options where you can go outside of the original 1980's design box that the engineers had to work with and you can do the best of both worlds. Bigger aluminum calipers of the aftermarket today can do the performance job that our smaller OEM calipers of the day limited to the size constraints could not, it's just that simple. If you want to stay OEM-ish, do it, more power to you. No one will knock you if that's what you want. Could get more bang for the buck going with some of the other methods and setups mentioned within this thread however. As I said in the start: What do you want to do with your car? What is your performance goal? Circuit? Drag? Street? Stock/sleeper? You need to decide that before sinking money in it.
1992 Nissan Skyline GT-R Gunmetal Grey - Long Live the Z and Godzirra! - "Love cars. Love people. Love life." - Mr. K.