The standard zoom focal range is one of the most popular areas for lens development, so the fact that Canon now have three L-series models is no real surprise. Their overall DxOMark performance shows that in this case, price is quite a good indicator.
In the sharpness scores, the results are the same, the EF24-70mm f/2.8L USM is clearly the superior lens again followed by the f/4 version and very closely behind that, the 24-105mm.
Interestingly, in both the transmission test and the distortion test, the EF24-70mm f/4L IS USM outperformed the other two models. In the case of transmission, this is relative result to its stated maximum aperture, but for distortion, it is an absolute result.
In vignetting, both the EF24-70mm f/4L IS USM and EF24-105mm f/4L IS USM lenses score a rather poor -1.3EV, but the more expensive f/2.8 model is even worse, surprisingly only scoring -1.7EV.
The chromatic aberration comparison is also interesting, with once again the most expensive lens coming at the bottom of the pile, while the EF24-105mm f/4L IS USM comes out on top with a score of 6µm.
Looking outside the Canon stable, the Sigma 24-70mm f/2.8 IF EX DG HSM Canon mount lens is very worthwhile contender, out-scoring the EF24-70mm f/4L IS USM lens in sharpness and transmission, and being very close in the distortion tests too. It’s only really the vignetting and chromatic aberration tests where this lens falls down (a score of 29µm for chromatic aberration is really quite poor), but the price is substantially lower than the Canon model, which may help you overlook its shortcomings.