Which subject to choose in M.Pharma

That really depends on your interest. For your whole life you will have to be involved with the subject you choose to study in M.Pharma. Thus my suggestion will be to go by your interest.
Recently one of my friends got selected in Bhim Rao Ambedkar but he was getting Pharmacology there. The fee in the college was only about Rs 1000. But he left the college and is pursuing M.Pharma from B.B.D Lucknow in Pharmaceutics where fee is in lakhs. He did so only because of his interest as he had interest in Pharmaceutics.
Again a lot depends on what you wish to do after M.Pharma. In pharmacology you get an additional option to join the emerging field of Clinical trials. Moreover you also become eligible for the post of lecturer in medical colleges when you choose Pharmacology in M.Pharma.
Pharmaceutics has an advantage that the students of pharmaceutics are given more preference in production units, R & D and F & D.
Similarly you may opt for Pharmaceutical analysis, Pharmacognosy and other subjects depending on your interest.