Shield material doesn't matter - if the shield skill activates, a shield will block all damage, regardless of material. You can make a shield out of balsa wood and it works like cotton candy.
In fact, you might as well just go for something light, and maybe something that won't burn or melt.
In terms of real-life properties, the DF version of iron is pure iron, which is idiotic to bring into battle because if weapon deformation were modeled, that would mean your swords would bend like clay in combat.
Real-life crude iron weapons were actually more often crude because of having too many impurities that were never removed from the iron ore than having no impurities at all. This meant that crude iron was often brittle and had to be cold-forged. This, in turn, meant that crude iron weapons (like those of more common soldiers on the medieval battlefield) were actually commonly shattering when striking a hard enough target, like a knight in full steel armor.
Bronze, comparatively, makes lower quality weapons than steel, especially, but bronze armor, in spite of being softer and more malleable than steel, is more useful in combat than crude iron.
Hence, if you're having an argument about which would have been more useful in real life, then the answer is that bronze is superior to crude iron, but inferior to well-made steel. It's all a matter of what quality of metal you can actually produce.
DF is a special case because it tries to model a lot with its various metrics, but several critical pieces of real physics are missing - weapons and armor don't deform and merely shatter (which is why creatures made of fire are like creatures made of tissue paper, and can have their fire "broken" by a stiff wind) only to be returned to whole for the next attack for the purpose of blocking damage. It also doesn't have any sense of the concept of the purity of the ore or what impurities do to the chemical properties of a metal. All ores just produce pure metals because that's the simplest thing to model.