Perhaps the older lens has more pleasing colors:
normally absorbing a bit more of hi-freq(blue/violet) lens a comfy "orange sunset" effect and warm atmosphere.
Modern digital cameras tend to be "colder"
in color, with sharp blue/violet.
To me, the copy of the EF 1.2L that I tested had a softness to its rendering that was very painterly. I shudder to use the term "microcontrast", but the lens appeared to have solid contrast and sharpness at broader spatial scales and a softness at the smallest scales.
It felt like a lens that rendered every scene with a deft touch, making everything just a little prettier than it might really be.
Someday, I may find a used one to call my own, but I can't quite justify its weight and expense today. The EF 1.4 largely fills that bill without the price and heft, but doesn't render things so instantaneously as art.
Perhaps the older lens has more pleasing colors: normally absorbing a bit more of hi-freq(blue/violet) lens a comfy "orange sunset" effect and warm atmosphere. Modern digital cameras tend to be "colder" in color, with sharp blue/violet.