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As far as the scopes go, the main difference between the ED100 @ 900mm and the ED120 @ 900mm focal length is that the ED120 will gather more light with its extra 20mm of aperture.
So *if* you have really good seeing without atmospherics getting in the way, you could use higher magnifications for the planets, and Deep Sky objects should be a bit brighter in the 120 vas the 100.
Otherwise to my (limited) knowledge they are effectively the same. Same focal length the 120 just lets in more light.
If I had the money, I'd go for the 120.
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