Which is the better substitute for tapioca in Chinese/South East Asian dishes, e.g. for thickener of soups and for meatballs? Is it corn starch or potato starch? I am asking because both corn starch and potato starch are more readily available in general supermarkets, whereas tapioca is a bit harder to find.
Both potato and corn starch would work equally well in the preparations you describe. When substituting flour, the proportions are equal, e.g., one tablespoon flour to replace the one tablespoon of tapioca. With cornstarch, it would be less: i.e, one tablespoon of cornstarch per two of tapioca. Were you to use pearl tapioca, it would be two of soaked pearl to the one of the quick-cooking version.
If it's available, it may be better to replace the instant tapioca with an equal amount of arrowroot starch. Both tapioca and arrowroot contain the same kind of starch, amylopectin, which differs from the amylose found in flour and cornstarch.