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Hi Benn,
The simple answer is to square the radius of the apertures. 64, 81 and 100. Assuming they are all used at similar magnification, in the same conditions have similar quality optics and similar contrast elements.
A 16" 'scope will produce 64% of the light at the focal plane compared to the 20".
An 18" will produce 81% of the light at the focal plane compared to the 20".
The practical effect in my experience is a little different. For many years I regularly observed with two friends and the three 'scopes were: 16", 18" and 20". The consensus among the three of us was that going from 16" to 18" didn't make an incredible difference. Neither did going from 18" to 20".
But, going from the 16" to the 20" was. That said all three are good aperture choices and provide lots of bang for your buck.
The best piece of advice I ever heard about choosing a telescope is this: The best telescope for you is the one you will use the most often.
When you do feel like a bit of astronomising, there's little point in owning a telescope that says: "Oh it's just too much bother".
The one you want, when you feel like astronomising is the 'scope that says "Pick me up, take me outside, the reward exceeds the effort expended."
You pick the one you believe you will use the most.
Best,
L.
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