I have a 6D and use a 24-105 L as my walk about lens. I also have a 17-40 L, but as nice a lens as it is (particularly when stopped down to f8 and smaller), I use the 24-105 a lot more often. How much you will use a certain focal length will depend on the type of photography you want to do. Perhaps have a look at the XIF type details on some of your favourite photos and see what focal length you actually used. This will work on other people's photos on Flickr too if they've not turned off the details, so maybe have a look what focal length any photos you like on there have been taken on?
Also, you don't say whether you're planning to buy used or new. If you were thinking of new then maybe think about buying used instead? You could you perhaps get a mint-ish Mk 1 24-105 L IS (not all
that different from the Mk 2 in terms of image quality)
and a similar 17-40 L for about the same price as a new 24-70 f4? If so, I'd be tempted to buy the 24-105 from a long-established and reputable dealership/shop with as long a guarantee as you can find, that way if the aperture ribbon cable breaks (which can be an issue on some of these lenses) you should be covered for the guarantee period.
I hope my suggestions are useful and give you some options to consider. However, it's about deciding what is right for
you, not me or anyone else who may give you some ideas/suggestions, so best of luck deciding.