FWIW, and in my opinion, the older f5.6 "S" version was more of a "true" macro, err, "makro." Meaning that it was designed to perform at its best at a 1:10 distance, while being only mediocre at farther distances. The more modern version of the 120 (f4), as with other brands like Nikon, while still being a "makro" lens, has been designed to be more of a universal lens, with overall performance improved at infinity settings, but at the expense of some makro performance.
For me, a macro lens is chosen for its performance up close. And as such, I'd opt for the older 120/5.6 S. Today, the 120/4 version is often used as a short portrait lens, and it probably performs better in that distance range. The 120/4 may be a better overall performer, but I doubt it can ever match what the 120/5.6 S could do in the close-up range.
Same with Nikon. I'd take the 1970's 55/3.5 Ai or 105/4 Ai versions over the newer f2.8 auto focus versions any day of the week, if "macro" was my only intent.