We had two copies of the 24-70 mk1. One was quite a bit sharper than the other but no match for the mk2. I like that 24-105L, just a bit slow and not in same category as the mk2 24-70 when it comes to sharpness. I'd have to look at my lens tests to know if the 24-105 was sharper than my sharp 24-70- either way it was close and the 24-105 has IS which is nice (but f4 isn't ideal for what I use it for).
The new mkII version is noticeably sharper than the sharpest mk1 lens we had especially wide open. The mk1 was noticeably sharper at f4 than at 2.8 (even in center). The new one is almost as sharp at 2.8 as at f4, pretty amazing. The lens also produces warmer, more natural looking images. When I say sharper I was a bit blown away when I compared the images. Just looking at the leaves on distant trees the older lens was a bit blurry and the new one was tack sharp- looks like a good prime lens. Really can't go wrong with this lens except the price
Sorry, can't answer your question about contrast- don't have a great way to test it.
Coming back to the multiple copies we had of the Mk1... We replaced both with mk2 and both mk2 lenses were about the same sharpness so another argument for that lens is that you're more likely to get a good sample.
On the negative side I really don't like the stiffness of the zoom compared to the mk1. The mk1 felt better to me. Wonder if anyone else has an opinion on that.
That new 70-200 2.8L II is pretty sweet as well. Glad I waited 10+ years to refresh our lenses
I'll be kicking myself when Canon comes out with an IS version of the lens no doubt though. Until then I'm super happy