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What 20mm is it? If a widefield then your true field will be similar to the 25. I'd say keep the 20 and get a 32mm Plossl for maximum true field in 1.25" and nice bright images. There is a good 25mm Plossl for sale in the used section if you really want that focal length.
Note maximum true field is determined by the eyepiece field stop diameter. Max for 1.25" is 27mm and this is available in 40mm Plossl, 32mm Plossl and 24mm Widefield with 44 deg, 50 deg, 65 deg apparent field respectfully.
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