Used 5DM1 vs 50D

http://en.wikipedia.org/wiki/Crop_factor

Under section "Magnifaction Factor", it says,

A given lens casts the same image no matter what camera it is attached to. The extra "magnification" occurs when the image is enlarged more to produce output (print or screen) that matches a standard output size. That is, the magnification as usually defined, from subject to focal plane, is unchanged, but the system magnification from subject to final output is increased.

That's the reason 100mm lens on 1.6x crop to have the same FOV (or AOV) of 160mm on FF. So 1.6x crop does have "reach" advantage which is actually pixel density related. If you use the same 100mm lens on FF and shoot from the same distance, then you ended with a different photo with wider angle of scene. You will get the same scene if you crop out from FF photo to have the same FOV as from 1.6x crop. If others are the same you could have identical IQ from a future rumored 46mp Canon FF that has the same pixel density as from 18mp Canon APS-C. However if you use 160mm native length lens or moving closer with the same 100mm lens to fit the subject into the same FOV, then even 12.8mp FF will have advantage (fine details) thant current 16/18mp APS-C as it has 1.6x less crop magnification.

I will not be surprised that a 24mp APS-C will resolve more details than 12mp FF even fit the subject into the same FOV with a good lens. Crop can resolve more than FF but needs better lens and lots more pixels to overcome the side-effect of crop magnification or crop penalty.

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