I cannot answer your question, I live under moderately light polluted skies north of Melbourne and can just make out some clouds of the milky way when there is no moon present in the night sky. I've used a 5.11" dob (mine) and 8" dob (loan) side by side and the fainter objects appeared to pop out more in the 8".
You say that it will live on a rooftop terrace most of the time, so one question I would ask you is whether you have access to the terrace with an elevator or by stairs and if it is stairs then will you often have someone to help move the scope to your car?
As suggested above, I would recommend that you visit both a store and a star party so that you can see both sized scopes in person. From the store you will be able to handle the tube and base so that you can determine if you can comfortably move them about at home and to the car and at the star party you will, with clear skies, be able to compare the views presented by each.
Good Luck
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