>>2000736The material of the frame is irrelevant truly.
What matters is components, geometry/size, and your use case.
Generally speaking for used roadie's, a steel bike will be older likely with down tube shifter (new steel bikes are "boutique" and cost a fair bit sometimes). This means it's parts will be older too, which is fine but something to keep in mind for maintenance. It'll also be heavier, and probably have a more relaxed geometry. Rust is a concern but not a huge one, easy enough to check.
An aluminum bike will be newer, with newer parts. Modern standards, and generally lighter. No worries about rust on the frame but components can still end up clapped out.
It depends what you want. I've noticed, generally speaking, that a 2010's aluminum road bike with a carbon fork is one of the best bang for buck buys lately. For 300-800 you can get a nice rim brake bike with mid tier components and lots of life left. No crazy standards, easily replaceable parts.
A used steel bike can be very cool too, but they're often overvalued. A mid tier bike in the 90's is objectively a slower heavier bike than one from the future, but if the price is right you can get a LOVELY rideable bike.
IMO if you want performance/value you should go more modern aluminum, if you want a tough soulful beater you should go 80's/90's steel.