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I just went for the medium... easier for me to get into tight spaces. You'll still probably do the same number of passes anyways. Really up to you... no difference in how it will clean.
Word of caution on mag cleaners. Make sure there isn't any sand or hard particles stuck in the cleaning surface of the mag cleaner. It will scratch your tanks glass I know from experience.
having used 2 sizes of mag-floats, I decided the Kent pro scraper with the plastic blade is a better cleaning too. for coralline on glass, i just use a razor blade and a steady hand.