Forget weight for a moment, cheap tubing generally got less care in the build process than 531 (or other top tier tubing) and the geometry was also almost always different, but if one was to fabricate a "gas pipe" frame with the same level of care as a typical custom 531 frame aside from weight, one probably could not tell the difference, if two examples were built with the same geometry. While it is harder to "test ride" a steel lugged bike today, try to ride as many as you can and see what you like.