Sago pearls are derived from pith of tropical sago palm trees whereas tapioca pearls are made from cassava root starch and is called Sabudana in Hindi.
Sago is softer and gelatinious whereas tapioca is chewy and bouncy due to starch content.
Sago requires soaking and rinsing to remove starch and it takes time to cook while tapioca doesn't require prolonged soaking and cooking.
Sago pudding is mild and neutral and it absorbs the ingredients, tapioca pudding gets subtle earthy undertones with cassava starch.
Sago is used in soups, drinks like boba tea, and cakes. Tapioca is used in baking and thickening.