The best place to see the two words contrasted, in my own view, is Galatians 1:7 and 8 where Paul calls the false gospel being preached at Galatia as hetero.
He says that that gospel is 'another' (hetero) gospel but is not a 'similar' (allo) gospel.
It is another kind of gospel which is not a valid alternative to the gospel.
Thus hetero is of a similar category of existence. But allo is a close companion.
Thus in Matthew 16:14 the comparison is widening from those allo who - correctly - saw that Jesus has strong connection (see Malachi 3:1,2 - Messenger of Preparation and Messenger of the Covenant) to John the Baptist. Then 'other' saw a connection (understandably) with Elijah (who prefigured John) but 'others' hetero, making a much wider comparison and being wider of the mark in their categorisation, suggested Jeremiah or (even more vaguely) 'one of the prophets'.
The list widens from a wise interpretation to an obscure reference to a merely vague suggestion.
Thus those who speak these alternatives are categorised according to the character of their concepts.