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The OP has a 6” SCT not a 6” refractor and his neighbour has a stock GSO 10” Newtonian, not a ‘good’ 16” or 18” Newtonian. (Nothing to say the SCT or GSO aren’t ‘good’).
A more interesting comparison would be a 6” f10 SCT vs a 6” f5 Newtonian vs a 6” f8 Newtonian. All have the same aperture but the SCT is a closed tube. The 6” f10 SCT has 37% obstruction (by diameter which refers to contrast), 6” f5 Newt has 30% and 6” f8 Newt has 20% obstruction.
Anything over 25% obstruction by diameter has detrimental effects on contrast when viewing visually and it is noticeable. However the effects of thermal instability is just as detrimental.
If the OP ever gets a chance I would recommend he compares a 4 or 5” APO refractor with a 6” SCT and see what appears ‘crisper’ to him. The views in the smaller refractor are stunning but to get the same magnification the view will be dimmer. Is there any more detail visible in the smaller aperture? The rules for determining resolution would say no. Is the view ‘crisper’; often it appears so.
Now to get the same magnification in all ‘scopes mentioned one would have to use different focal length eyepieces of equal quality on axis if tracking or across the more of the field if not tracking.
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