Why can't we take them both. If they are both on the board, I would have no problem with both of them. The big men at that point in the draft are iffy anyways.
But if I have to choose, definitely Ross.
Harkless has a bit more upside, but he also has much more downside.
I think Ross' floor is as a quality defender/shooter in the Pietrus mould (although smaller). But I think he could be MUCH better. His release is very quick, and he has shown a pretty strong midrange game to go with it, as his handle has really improved. And defensively, he makes an impact. He is not a shutdown defender (at least not yet), but he is a pest. If he can continue to improve off the dribble, he could be one of the better SGs in the league.
Harkless on the other hand is just unproven talent. He has a lot of work to do to even be a halfway decent NBA player, let alone a star.