1.2 because it is more manly and built like a tank AND it's a frustrating feck of a lens to use
It's my current "project" and never off my MK3.
When you get a great shot with it, it's truly great and melts like butter.
When people say it's not sharp wide open, they mean it's not ALL sharp wide-open
The eyeball will be but everything else starts blurring from that point to a beautiful creamy background.
It's like shooting vampires.
I have the 1.8 and have seen the results of the 1.4 on a MK3 - the 1.2 is very different.
If you buy a 1.2, it's for shooting wide open at 1.2. If you find you are stopping down to 2.0 or more, save your money and get the 1.4 or the 2.5 macro.