[MrB]

The question seems difficult, if not impossible, to answer because it is asking for a comparison of different technologies - the glass is in the lenses, and presumably the same quality lens could be fitted to the three different bodies. The comparison is not then about the glass but about the combined performance of the sensor, electronics and programming within those different bodies. Or then another interesting question might be - does a higher quality lens on the 600D give sharper images than a lower quality lens on a 6D?

Cheers.
Philip