I directly compared the 24L II to the 24-70 II at 24mm at comparable apertures and the zoom had better IQ. Granted, its close... But its amazing that the zoom was able to beat the prime.
Well either I have a stellar copy of the 24L II or the 24-70 II I used was a lemon because that wasn't my experience at all, but it was close. In fact my 24L II is one of the sharpest lenses in my kit and I think I have some pretty decent lenses.