I'd much prefer the 24-105mmL for way less, they often pop up for $650. The longer focal length range makes up for the very tiny difference in MTF. I have the 24-70 f/2.8 MK II.
You sound like someone who has has never actually tried the 24-70 f/4 IS. I've tried all three (24-105,24-70 f/4 IS, 24-70 II, more than one copy of each too) and the 24-70 f/4 IS is much closer to the 24-70 II at 24mm than to the 24-105, the difference is not very tiny at all, much sharper edges and corners, much less distortion, much less prone to smearing purple fringing all over branches against bright white clouds, much less lateral CA (and as already mentioned much less longitudinal CA, although as not as much less as the near APO 24-70 II).
I could never tolerate the 24-105 for 24mm finely detailed edge to edge landscapes on FF but have no problems with the 24-70 f/4 IS at all (nor the 24 1.4 II or 24 T&S II or 24-70 II). The 70-105 can be covered at much higher quality (and with more more extra focal length as well) with a 70-200 f/4 IS or 70-300L (although it's a personal thing, for me the switch over at 70mm is fine, but I suppose there could be some for whom it would be bothersome).
Granted now that the 24-105L is only $650 or so, it's not bad for the price (I always thought it was an optical rip for the old $850-1250 price), but that still doesn't mean it's not more than a very tiny difference compared to the 24-70 f/4 IS, at least not if you care about the wide end, only at 50mm IMO are they similar at all (the weak point of the 24-70 f/4 IS is right around 50mm, it gets stronger below and above that focal length).
24-70 f/4 IS is also smaller and lighter than the 24-105 and has much more macro ability and a bit more effective IS.