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Yeah, agree with that...they're a bit hard to compare. I have a 24-70 II and sold my 24L to fund it. It's a great, great lens but I tend to use 24mm lenses stopped down (for low light shots I take the 35L).
Did some testing and I can confirm what saneproduction said; at 2.8 they are about as sharp but the zoom has some vignetting and distortion, although both can be quite easily fixed in post.
However, if you look at mid-contrast, colors, etc. I really think the 24-70 II is a prime killer, I use it most of the time now.