my suggestion to you....dont ask questions like this...you will never get a straight answer.
If there are 100 members on PW....you will see an incredible bias (which is natural for humans) for one against the other.
suppose majority tells you to get mehran, and since you are buying a used car, and if that used car you buy give you problems which is absolutely natural, you will second guess your decision. having said this, that mehran can turn out to be a great and well mantained car which wont give you any problems at all.
i would go to a reputable suzuki dealer or will talk to ppl i know personally who own mehrans about common problems. perhaps u can ask PW members as to what are most things to look for when buying a 1)mehran 2)alto or khyber.
the point is...there is no certainity when buying a used car. it can turn out to be too problematic for your liking.
good luck