Real differences are rulebook requirements for pull test, flying wire diameter in some classes, connector button size requirements.
Cannot speak for anyone's units currently made but am reasonably sure internal unit wires differ in diameter, .010 vs .012, and total number of internal wires. The units in my airplanes I know have .012 dia internals. I know the maker.
My main concern with the older units is not really knowing the condition of the wires after near to 35-40 years of being in the package. Some I have cut open out of curiosity have looked rather bad as to the wire integrity. No real way to flux the solder joint without leaving some bad stuff. A couple have had the solder sweat its way up inside the unit an inch or so. This changed the effective length of the internal wires, reducing the amount of motion.
Career wise, I have had one unit failure. It was on a Kansas Twister 60 airplane that I had purchased from an well known builder. First contest with it and a K&B 65, it failed the pull test rather spectacularly . Unit pulled apart and ripped the wing open to the tip. Looking at the remains showed the internal wires to be very well consumed by whatever solder fluid he used.
Did a new wing and made my own unit and later ran a 201 with it. Obviously this was before the current fuel rules.
Having a nice new build fail the pull test because of an old unit will be depressing.
Think about it.
Luke