Answer
$AsO_{4}$ $^{3-}$ is the stronger base.
Work Step by Step
$AsO_{4}$ $^{3-}$ is the stronger base. Since they have the same number of oxygens, the comparative strength is dependent on either P or As. As is less electronegative than P, meaning that the charge won't be as strongly held, and is more "open" for hydrogens to attach, making it a stronger base.
