I think if you compare their overall strengths and weaknesses, Azhar is the superior batsman. At no stage of his career was Misbah ever good enough to score a double-hundred in Australia.
However, Azhar has been badly exposed once he was burdened with the responsibility of being the senior batsman. I really thought he could have averaged around 45 as an opener for the next 4-5 years, but that doesn't seem to be the case.
He is mentally very weak and has failed to compensate for his ageing reflexes and added responsibility. I really don't think this is just a slump in form; he looks spent, and with the next series 8-9 months away, this is the perfect time to thank him for his services.