Having just done this, my advice is: buy the kit. I didn't, and within a short time, I ended up buying the 24-105 separately, for more money.
24mm to 28mm the lens exhibits very strong barrel distortion and heavy vignette.
These are trivial to fix in post. In Lightroom, you check one tick box. The optical issues to weigh heavily are things that are not trivial to fix in post, such as softness.
I have the 24-105 on my 5DIII more than any other lens. It is an extremely useful focal length range. A couple of the 24-70 options are optically somewhat better, in particular, the far more expensive Canon 24-70 II, but I find that the extra range and IS more than compensate for me, using it as a walk-around. I have saved shots with that extra 35mm that would have been tough or impossible without it. Re the barrel and pincushion distortion: it does become apparent with some kinds of shots, but when I notice it, I simply check the lens profile box in Lightroom.
I didn't buy it as a kit because, like you, I was weighing all the alternatives. I am still kicking myself for not having bought this as a kit.
Off subject a bit: enjoy the 5DIII. It is a truly great camera. It has a lot of very useful customization options, so give yourself time to work through the manual. The manual is generally quite good, except that the section on the AF system is a little too brief. I had to play with that for a while to figure it out. It's worth it. The AFT is quite amazing. You can tell the camera to use a portion of one spot, one spot, one spot assisted by either 4 or 8 surrounding spots, a zone, or the entire 61 points. 41 of them are cross-point. The camera lets you set different points for the three different orientations, and it will remember them. You can also register this (and other settings) to one of three custom settings. For example, I have one of the custom settings set for my indoor flash default: 1/60, f/5.0 I set this to use a single point that is about 1/3 of the way from the top in either orientation. So, in either vertical or horizontal orientation, I have a single point set at about eye level for most shots. I have the camera customized so that if i want a different point, I can just flick the joystick on the back with my thumb. And a nice bonus: the camera makes use of the large number of AF points to give you an internal level in the viewfinder, which I have programmed to turn on with the little m-fn button near the shutter. Somebody gave this a LOT of thought.