Hi Shaun,
This subject can be as easy or complicated as you like it to be. I asked myself the same thing some time ago and read a lot here and elsewhere about how many times more light a bigger scope would gather (a misleading approach because 4 times more light - just as an example- isn't 4 times brighter), to buy the biggest aperture you could afford (an unergonomic approach because weight increases faster than aperture as you go bigger) etc. etc.
To me, the simplest way to compare two scopes in practice is this (using the numbers of your candiates): 12"/8"=1.5
This means the bigger scope will show the
same object at the
same apparent brightness in the eyepiece 1.5 times bigger.
Yes, the bigger scope resolves finer detail.
Yes, the bigger scope sees fainter objects.
Yes, the bigger scope looks more impressive.
But the first consideration above I found
by far the most tangible. I know it's simplistic.
I have a 2.5", a 3" and a 10" scope. I can make all of them show the same object at the same brightness
