I guess i'd recommend Urdu because
A) If i'm not wrong, arabic uses same script as Urdu
B) If you speak Urdu, you could understand hindi ( not that its gonna help you in Afghanistan but still, from a personal development standpoint, it would )
C) The Peshtu (Sp?) language has roots in Urdu too.
Since some of the local languages evolve from Urdu, learning Urdu would be beneficial imo.