Some interesting replies; thanks guys
I think at the end of the day, IQ is one of my main considerations but handling/enjoyment of using the camera is important too, hence my dilemma.
I should have mentioned that I had a 6D/24-105 combo for a couple of days before returning it - I reckon I got a poor copy of the lens as it was incredibly soft in the corners at F4. However comparing shots between the 6D/24-105 & 100D/17-40 showed no discernible difference except that the 6D/24-105 shots at F4 were much softer in the corners.
Another 'fly in the ointment' with the 6D was what standard zoom to go for - the 24-105 F4L I tried didn't impress, the 24-70 F4L reviews I've read suggest is only marginally better (& some reviews mention focus-shift) which leaves the 24-70 F2.8L MKII which does
look nice, but very expensive...
Whereas I probably don't absolutely need the AF performance of the 7DMKII, it would be nice to have. Not too worried about shallower DOF with FF & as to 1.6 crop factor, good for some scenarios not for others.
I think I'm going to wait until the 7D MKII has been out for a while to see what real life user reviews are like but i'm expecting the sensor to be the best APS C produced to date by Canon & with Digic 6 too I don't think there's going to be much between the two cameras IQ, at least that's what I'm hoping