I spent my morning pixel peeping! Im no expert at this, I was doing it for my own benefit but decided to post my findings...I'm only posting a small fraction of all the tests I did. I tested various focal lengths etc, but the tests were similar throughout. Here are my findings:
24-70 slightly sharper @ f4, identical from 5.6 onward
24-105 very slightly better colors
24-70 noticeably less distortion
If you were taking normal pictures, these lenses would be absolutely identical....if you want 2.8 get the 24-70, if you want IS, lighter weight, longer range, and cheaper get the 24-105. I'll be sending my 24-70 back and keeping my 24-105.
24-70 pictures first
24-105 pictures second
Sharpness:
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Distortion (look on the left hand side):
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