I explain in my mango sago recipe that even though sago and tapioca are used interchangeably, they are actually different things in that sago is an edible starch that is made from the pith of an array of tropical palm trees while tapioca pearls are made with tapioca or the starch from cassava, a root crop. However, they look similar, taste similar, have a similar chewy texture when cooked, and cooked the same way. They’re often substitutes of each other in recipes too, though not always.  You can use either one for this recipe.