Re: IQ of Canon 135mm f/2 prime on a 5DII VS 5DIII??
In short, no. When the shot is well focused, I find no tangible difference between electronic view or print between the lens on either my Mk III or Mk II.
The lens plays well with both camera's focusing systems, with the well-known Mk II AF limitations being no worse or better with this lens.
The Mk III is more effortless focusing with this lens with the wider, and more accurate array of focus points, allowing off-center composition without focus-recompose technique that can be an issue with super-shallow DOF capable lenses like the 135 L. Nevertheless, understanding the Mk II focus system and it's limitations, I've never really had an issue getting the shots I want out of that combination with the 135 L.