I think they are comparable. A blow from a bronze sword will kill you just as well as a blow from an iron sword. The most widespread weapon of all times prior to the American Revolutionary War and the Napoleonic wars were spears. A spear with a bronze tip and one with an iron tip will also kill you with equal probability. If you have a shield, the damage to the spear is much more likely to come from a broken shaft than from a cracked tip. The shield itself generally had a metallic part (a "boss") that could be made of bronze or iron. Both were equally effective.
Swords were generally not used in battle. But if used, a bronze sword was a bit more likely to crack than an iron sword. Bronze weapons were resistant to corrosion though. So that meant they did a better job in their secondary role: that of provider of status. A warrior with shiny armor, shield and sword is much more imposing and frightening than one with a rusty armor, shield and sword. Of course, in the bronze age, war was more ritualistic than war during the Roman empire, when war was much more like an industrial operation. For the purposes of the Roman empire, iron was probably doing a better job than bronze.
People often compare sword with handguns. Handguns nowadays are deadly weapons, but they are not used in war, where people use assault rifles instead, or other heavier weapons. Swords were sidearms. Warriors carried them around every day, they could be used for self defense (in those times there was not police or law enforcement). Or they could be used for intimidating opponents, and if needed, for chopping someone's head or limb.
In combat though, spears were much, much more effective.
With one exception. As far as I can tell only one exception, but a big one. The Romans have figured out that all other people are quite bad at close quarters combat. Since soldiers were used to using spears, very few soldiers knew what to do if someone got in a close range than the length of a spear. The Romans then would bull-rush the enemy using their shield, and when close they would use their (quite short) swords. Without training, their opponents didn't stand a chance. Of course, this type of battle tactic required an enormous amount of training. The Romans did that because they were a very war-oriented society. Others couldn't match their level of training.
Even for the Romans though, who were regularly using swords, the choice of bronze versus iron would have made little difference. Although bronze is a bit more brittle than iron, and a bronze sword is more likely to break when it strikes a hard object, the Romans were trained to thrust the sword, not to use slashing moves. Thrusting is much more deadly, but less instinctive. It's another thing that requires extensive training, which the Romans were great at. For thrusting, I think a bronze and an iron sword do equally well.
Nice explanation, but the Romans did use spears - the 7-foot heavy pilum. They just threw them. It was effectively used to disrupt a threat in a mass volley before swords were drawn and the zerg rugh. Several battles were won by the pilum - the battle of Telamon for example.
Swords were generally not used in battle. But if used, a bronze sword was a bit more likely to crack than an iron sword. Bronze weapons were resistant to corrosion though. So that meant they did a better job in their secondary role: that of provider of status. A warrior with shiny armor, shield and sword is much more imposing and frightening than one with a rusty armor, shield and sword. Of course, in the bronze age, war was more ritualistic than war during the Roman empire, when war was much more like an industrial operation. For the purposes of the Roman empire, iron was probably doing a better job than bronze.