If we werent to buy a RB and had to choose between the two, i think id have to say Richards!
Im not Richards biggest fan but i dont think Zabaletta is a good full back! he is much better in midfield as the defensive mid role suits his style and he will be a good back up to De Jong if Hughes uses De Jong as the defensive mid player as i think he should!
But it's a difficult one because Richards has many floors to his game! I think because of his poor control and dribbling ability he is better suited to centre back, but i think we will be signing big in this position and Onuoha,who will be ahead of Richards due to last seasons improvements, will even be on the bench!
Its a tough one! and even though the OP said dont put that we need a new full back, ideally we do need a new full back! Maicon has been linked but i think he's average, Johnson has gone to Liverpool which im disappointed in, but im sure there will someone!