We need someone who can reverse-swing the ball. Mohammad Asif relies on seam, and Mohammad Aamer hasn't learnt reverse swing yet That leaves us with Gul or Sami. My vote is for Gul, because he can not only reverse swing, but also bowl good yorkers.
He hasnt learnt it fully yet but he is learning from Waqar first hand. Evidence is second innings at Melbourne when he got three quick wickets through reverse swing :)