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My main planetary scope for many years was a C5. As you allude, it's resolution is to the typical limits of Sydney at sea level.
I picked up a C8 about a couple of years ago. This allowed me to compare the two scopes side by side. In the end I let go of my precious C5 as the opportunity the added aperture gave on those occasions of great seeing made the C8 the better option. Yes, on most occasions I can't go beyond the magnification the C5 gave (250X), yet enough opportunities do happen that make 400X a practicality. Aperture here won out.
With my 17.5", seeing conditions also are the limiting factor to magnification. Then what matters here is the light gathering grunt that AG mentions. At the same magnification between a C8 and the 17.5" (easy to do as they have the same focal length), the difference in image is so significant that making a decision of limiting scope size to prevailing conditions would have been a very poor choice. Aperture always wins, regardless of conditions. So, conditions won't allow for much magnification, big deal really. Magnification really isn't what it is all about, is it. Tell me that there is no difference in the image quality between a 4" aperture and a 17.5" at the same magnification, and you really haven't experienced the difference.
Wanna see the difference? The two sketches below were done from my home in Sydney's eastern suburbs. The one on the right was done with an 8" f/4 Newtonian (not the C8). The one on the right a 17.5". Yes, there is a magnification difference - same EP was used with both scopes. But the difference in the brightness and visible detail of each depiction of M42 shows the advantage of aperture, regardless of prevailing conditions.
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