8" under dark skies will easily beat a 12" in moderate light pollution. The difference between these scopes is just under 1 astronomical magnitude (mag). Difference between a dark site and the outskirts of a large city is 1.5-2 mag. So you are still 0.5-1 mag behind at the polluted site even with the bigger scope.
The only exception will be planets and the moon at high powers in good seeing, where the object is so bright that light pollution does not matter at all, and the resolution of the instrument (as limited by the aperture) will be the most important thing.
From personal experience I can tell you that an ED80 (80mm aperture) shows more just outside Marysville 80km from Melbourne CBD than the 8" GS Dob (200mm aperture) from my back yard 20km from the city.
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Originally Posted by ballaratdragons
Last Camp, Steve (Janoskiss) abandoned his GS 8" and commandeered my 12" with accompanying "oooh's" and "aaaahhhh's".
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Side by side the difference is remarkable although might not be as great as you first think looking at the diff in physcal size between the scope. I did go and buy a 12" Dob shortly after that session with Ken's scope. I do still use the 8" - more often than the 12" actually; but that's because it's easier to lug up and down the stairs and because for observing planets in average seeing the 8" is sufficient.