I have both of them.
I did some test shooting today and I believe the optics are exactly the same. I can't tell the difference in the photos, so much that I don't know which one shot which on my test photos. They have the same color saturation, but I think the Mk.I is brighter in the corners, I will check this again later. with a proper test using a tripod. I think the only difference between the two is metal amount and superior build quality of the Mk.I, as well as it's distance scale. And the AF on the Mk.I feels a little quieter.
All testing was done on a Canon 5d full-frame.



