Upload & Sell: Off
From several test series comparing the two lenses I can see that they are set up differently, neither could be said to be a poor copy.
At F2.8 the older copy is clearer, sharper more contrasty than the later, but by F5.6 - 11 the newer takes over as slightly sharper and clearer, then at F16 - 22 the older takes over again! (This seems to be across the frame, but I'll do more tests to verify that.)
(NB. From F3.5 - 16, and at 35mm, my Contax Vario-Sonnar 35 - 70mm, still lives up to its reputation by slightly bettering them both, in resolution and clarity/contrast!)
If the results were shown in graph form the sharpness of the older would be an almost flat graph, just falling off to diffraction from 16. The newer would be a dome shaped curve that rises above the line of the older between 5.6 - 11, but below at each end.
Whether this is a definite decision by Canon to set up the later lens differently, or just individual copy variation is impossible to say. (Perhaps when I've sold one, I'll look out for another, for another comparison?)
However, as I generally shoot landscapes at F11 - 16, (because of needing to focus lots of different heights), and prefer the clearer focussing at F2.8 of the older, I think I'll sell the newer and keep the older.