Due to inert effect, +3 oxidation state of Bi is moer stable than its +5 oxidation state while +5 oxidation state of Sb is more stable than its +3 oxidation state. Therefore, Bi(V) can more easily accept a pair of electrons and get reduced to form more stable Bi(III) than Sb(V) can accept a pair of electrons. In other works, Bi(V) is a stronger oxidising agent than Sb(V).
