Check out the other posts in the series: Canon EF 24 70 f/2.8 L v/s Canon EF 24 105 L f/4 IS L, Top 6 reasons why buy the Canon EF 24-105 f/4 IS over the 24-70 f/2.8 and Conclusion: Canon EF 24 70 f/2.8 L v/s Canon EF 24 105 L f/4 IS L.
1. You are most of an indoor/low light action shooter
The most obvious benefit of the Canon EF 24-70mm f/2.8L USM over the Canon EF 24-105mm f/4 L IS USM is the extra f-stop. So if you find yourself shooting mostly indoor or in low light condition and you need that fast shutter speed to freeze the action, then you should be considering the Canon EF 24-70mm f/2.8L USM
2. You are a portrait shooter
The extra f-stop of the Canon EF 24-70mm f/2.8L USM and its smoother bokeh makes it a better portrait lens than is f/4 counterpart.
3. Selective Depth of Field
The shallower Depth of Field at f/2.8 makes the Canon EF 24-70mm f/2.8L USM a nice lens if you like to take pictures where just a small portion is in focus.
4. If you are most likely to use it at wide angle range
The extra reach of the Canon EF 24-105mm f/4 L IS USM doesn’t come free. The trade off is more vignetting and barrel distortion than the Canon EF 24-70mm f/2.8L USM in the 24 to 35mm range.
5. You like real closeups
Although not a macro lens, the Canon EF 24-70mm f/2.8L USM has a minimum focusing distance of 38cm vs 45 cm for the Canon EF 24-105mm f/4 L IS USM. I know the difference is not that important but it still should be pointed out.
6. Your camera has a 1.6x cropping factor
If you are using one of Canon EOS DSLR with a 1.6x cropped sensor, then this could also be an added value for the Canon EF 24-70mm f/2.8L USM and that for 2 reasons.
a. Depth of Field consideration: For the same aperture opening, the Depth of Field on a cropped sensor camera is greater than on a Full Frame. So if you like that shallow Depth of Field, you should go for the f/2.8
b. Reach consideration: The 1.6x crop factor could compensate for the extra reach of the Canon EF 24-105mm f/4 L IS USM
7. You already own or planing for a 70-200 lens
I am well aware that overlapping lens range is a bonus, not a deficit but if you already own a lens that can cover the missing range in the Canon EF 24-70mm f/2.8L USM and you find yourself falling in the category of most questions asked in this post, then maybe this idea will help you take the final decision.