Which rusts faster iron or steel?

Iron rusts faster in theory, but it all depends on the conditions. Steel is just Iron mixed into an alloy with Carbon and (sometimes) a few other metals. There are many different grades and types of both Iron and Steel, and they all oxidize at slightly different rates.

In short, oiled or finished steel will give you a fair amount of rust resistance, but it is always necessary to maintain metals to prevent oxidation.