The difference, by the numbers
Both have lemon juice. Only the Cinderella has orange juice, pineapple juice, grenadine and soda water; only the Virgin Mary has tomato juice, hot sauce, Worcestershire sauce, horseradish and Tabasco, celery salt and pepper. In parts, a Cinderella is 2¾ orange, 2 pineapple, 1 lemon and 2¾ soda; a Virgin Mary is 10 tomato and 1 lemon. By volume, the Cinderella is 86% mixer and juice and 12% citrus; the Virgin Mary is 91% mixer and juice and 9% citrus. That makes them close cousins on the ratio map.
They are about equally strong in the glass (both 0.0% ABV), and they hold about the same alcohol (0.00 standard drinks each). The Cinderella has more calories (about 76 against 31) and more sugar (13.6 g against 4.5 g). Both are served in a highball; the Cinderella is shaken, the Virgin Mary stirred.