Cost will determine whether I replace one or both. Logic suggests that ideally both should be replaced. In that event though I will still do the cracked one first and then compare it with the other old one. Due to time constraints this might not happen for a couple of weeks. Despite being cracked, the amount of movement does not seem unreasonable to me and I am not concerned that the mount is about to let go completely. It strikes me that the amount of movement I am seeing (and you are probably also seeing) is quite normal. I expect that these rubber mounts are designed to be resilient to isolate road noise and shock loads from reaching the body.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.