I have an M5 pro at work, and certain things are a smidge faster than my personal M1 Max, but they're pretty close overall.

Given how much more you can get buying used versus new, I'd go used unless there's some feature/characteristic only the new hardware has.

Having said that, I could see stuff slowing down over time with OS updates/etc. I've had my M1 air for a while, and some UI stuff was slower after specific updates (I think glass in particular).