I haven't brewed a fermented hard seltzer, but for soda I prefer citric acid for most fruit-flavored drinks (berry, citrus, stonefruit, melon, etc) and phosphoric for things that are more sweet/bittersweet (coconut, vanilla, coffee, etc.). I don't think I'd ever bother to brew a hard seltzer, but I'd certainly follow this rule of thumb as a starting point if I made one by force carbonating with water and clear spirits.