Originally Posted by
IthaDan
The difference, as I understand it, is the shape of the tubes- whether it's butted, if it has been extruded with or without a seam.
There are subtleties to steel frames, but it's more akin to the differences between varieties of apples compared to a true apples and oranges comparison like steel vs. aluminum.
There can be major variation in 4130... more so then Dan points out (But he does know his stuff). Case in point I own several mtb's one being a 1992 GT and another being a late 80's DB the DB is smaller and both are 4130.. but one weighs a lot less and rides a lot better.. hint.. it's the GT. The DB is like riding a lead noodle... night and day differences.
If the decisions between generic 4130 that you can't get enough detail about, or a frameset that's 520.. I'd pick the 520 every time unless I can lay hands on both bikes for a test ride.