According to WebMD, whether Tums or Rolaids works better depends upon the person and the symptoms targeted. According to the New York Times Health Guide, antacids with calcium may lead to constipation. The main side effect for antacids containing magnesium is diarrhea.
According to the manufacturers, Rolaids uses both calcium carbonate and magnesium hydroxide, while Tums uses only calcium carbonate. As RxList explains, antacids using magnesium hydroxide work more quickly but for a shorter amount of time than those using calcium carbonate, which dissolves more slowly after it is taken and lasts longer. RxList also advises against using antacids with large amounts of calcium carbonate over a long period of time.