I've owned the EF 24-105 and used the II. First one got stolen, glad to be out of that crime infested city! Augh.
But having owned the 24-70mm f/2.8 II for a while now and previously the old Sigma version the f/2.8 is more useful or than the 105mm for me with portraits. I think the 24-105mm range is excellent for a lot of other stuff though. As far as cropping, I normally just use the correct focal length for the job. But most times for portrait work I use my 85mm f/1.2 and the 24-70mm for looser shots and typically the f/2.8 is fine for that. Still need a fast 50mm prime really badly though for when I want more separation for other shots.
For landscapes though, 24-105 range was nice and I think the better general purpose lens if thinner DOF is not of prime importance. IQ I think is fine for both in my eyes, the way I used the 24-105mm for general purpose shots and landscapes ect. I was stopping down for DOF anyway. But that being said the 24-70mm is still sharper.
I have thought about getting the 24-105mm again in my kit it was still a much loved lens till it got stolen and didn't want to sell it. It is still a very nice and useful lens and cheap enough that it can be worth owning both.
I really like both the 24-70mm and 24-105mm lenses.