The tests I've seen show that the 1.4 excels at 1.4. The construction is apparently better than the 1.8 but sharpness is not improved in the 1.4, and in fact is excelled by the 1.8 at 1.8 to about f8, where they are then equal.
But here's a comparison chart from Ken Rockwell.
I would avoid the STM version because the barrell distortion is a bit stronger than the other two. But do compare the 1.8 II and the 1.4
The saving could put some nice filters on your lenses too.
http://kenrockwell.com/canon/compari...0mm-lenses.htm
In terms of sharpness, he gives this summary:
Sharpness
They are all equally super-sharp at moderate apertures, and get equally softer from
diffraction at the smallest apertures.
The only visible differences occur at the largest apertures and in the corners. In real-world shooting the corners never have anything in focus so it doesn't matter, but under laboratory conditions where we devise tests that have things in focus out there I can see what I'm going to share here.
The f/1.4 lens is the least sharp overall, but not by much. The f/1.8 and f/2.5 Macro are as good as each other, and better than the f/1.4, but the f/1.4 is the most popular because it's just about as sharp as everything else, and much more convenient and almost as light as the f/1.8.
If you can get the f/1.2 to focus reliably, when in focus, the f/1.2 is as good as the f/1.8 and f/2.5, and even sharper at the largest apertures, especially in the corners.
The f/1.0 lens is as sharp as the f/1.2 lens, and both are better than the others at the largest apertures. The f/1.0 focuses more reliably than the f/1.2, but is less sharp in the last few millimeters of the corners.
I broke my 1.8 and am looking for another Mark II
Trev