On my W123 (same vintage, different chassis) I went with the standard (black) Bilsteins when I rebuilt the suspension at 200K. They might have been Sachs-Boge...it was quite a while ago....I can't recall exactly now...
The heavy duty (yellow) Bilsteins are considerably more firm, and I was looking to restore the OE feel, not to stiffen it up. I've used HD Bilsteins (yellow) on a Ford Explorer and my Toyota 4 Runner. They were dramatically stiffer than OE...to the point that I took them off the Explorer, it was just too rough.
I've got HD Bilsteins front and back on my Volvo wagon right now, the T5 was the sporty version, and I installed the Bilsteins during a suspension rebuild that included new IPD sway bars. I like the overall firmer feel in that chassis.
Part of the W116 chassis appeal is the smooth ride, you might find the HD a bit too stiff...particularly if you've gone to a lower profile tire than OE...