And regarding the chart from Suiter's Book. This chart is accurate, but it ignores the eye contrast sensitivity threshold. A camera used in the 12" scope in Suiter's book would be able to exploit that contrast transfer at the higher frequencies, but the human eye gives up much earlier than this.
This is why when you see MTF plots, they will typically have something like "at the important visual frequencies" with reference to the chart. The eye simply can't see the lowest contrast, finest resolution detail that the scope can provide. For the case of a 10" vs 12" obstructed though, this level is considerably to the right and lower of the point indicated on the plots I have drawn.
Notice on those plots that the 5" scope would have a slighly higher threshold for visual use than the 6". Again, as has been stated, the transmission of the 6" would be reduced enough that in the real world, the 6" might not have the indicated limit, but if the scopes were 10" and 12", the threshold would be both to the right and below the threshold of these two scopes.
Again, CN dialogs almost never consider the benefit that luminance provides to the viewer. In my own experience, one of the greatest benefits of ever larger aperture (when it comes to planetary observing) is the tremendous gains in luminance, which improves the obsever eye contrast sensitivity. If you make no other change but to make the image brighter, the eye will see see the details more easily, and may see details in the brighter image that escape the dimmer image.
With that being said, neither of these would by my choice for a planetary scope, but my experience is that the MNs are quite excellet and hold their own against similar size refractors. They really are superb.