In brief:
The prenasalised pronunciation is older, as the spelling shows.
There are dialects that prenasalise and ones that lose the preceding nasal. E.g. Cyprus for the former, Crete for the latter.
Within Standard Modern Greek, prenasalisation has been associated with Puristic Greek, and un-prenasalised stops are becoming more common in casual speech; there are also associations with register, age, and class. This has been going on for fifty years. The cutoff Arvaniti & Joseph found in the linked paper was being born in 1955.