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You seem to be thinking about it visually rather than photographically?
The 10" f4 and the 12" f4 should capture the same amount of light over the same period of time as they are both f4. The difference is that the 12" will capture it from a smaller FOV with more resolution (if conditions permit). I think the decision should be dependent on the image scale and FOV you want. Practicality would come into it too as you alluded to in your post.
Hopefully that's helpful?
Pete.
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