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A 120mm will give you 44% more light gathering than a 100mm. This is more critical for visual use than for imaging. When imaging, if you have a good mount and good tracking you can image for longer and hence gather more light onto the image. If you are looking with your eyes, you don't have that luxury.
If I were you, go for the 120 if your budget allows. Otherwise, the 100mm will do fine for imaging but may be a tad disappointing for visual use.
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