They really cannot be that different IQ-wise, if you only want to look at detail retention. I have had many different generations of cameras, and I haven't seen a big difference in detail retention itself really, other than the 5D classic, which seems to have a great optical sharpness, which could explain why it still holds a special place in many people's hearts. If you want to discuss tonality, contrast, color, etc. then yes. Also with a very low contrast, low light situation, it is really hard to make really good assessment of the two cameras.
For example the same 100% shot from 2 cameras at the same exact exposure settings, they really show virtually no difference. I could post 100% pixel views and you would see a tiny difference, but not really anything that would translate to print. Now if there was sun, and color, etc. the differences would stand out a bit more perhaps. The detail would still be about the same, I expect, but you would see other differences. I chose this scene, because it has a mix of brights, darks, a bit of color, dull areas and areas that should show detail.


The image below is a 100% crop from a different set of shots using the L (just in case we are really seeing that cheap $250 f2.8 glass is the limiting factor). You should be able to tell a bit of which is which between the two at this point, but again, ff vs crop cameras don't seem to be so grossly different in retaining detail. Versedmb states that in post #20 as well, so we definitely agree there.
If you see major differences between the 5D2 and 7D using the same glass, there is something else afoot in my opinion. Either the 7D could be faulty (there are many that seemed to be sent in and came back better), or the lens needs adjusted to the body, or with the 7D density, you need to bump up the shutter speeds a bit more to stop motion more.
No post processing on any of these images, just resized, cropped, etc to fit POTN rules. And no, neither was taken with the S90...

Also, no, neither was taken with the 7D, it has a new happy owner. The 1D4 is off to Canon, so this leaves the XTi and 5D2.

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