The difference between them is that "347" is the area code of new york, including Bronx, Brooklyn and Queens in new york, while "9 17" refers to the area code of new york.
Extended data
The total area code of new york and new york is 14. In addition to the two mentioned above, new york has the following area codes:
2 12: USA-new york (new york: Manhattan only, overlapping with 646 and 9 17).
3 15: USA-new york (Syracuse, Utica, Watertown and north-central new york)
5 16: USA-new york (Nassau County)
5 18: USA-new york (Fort prats, Lake Saranac, Albany and Northeast new york)
585: USA-new york (Roster, Wellesville, Batavia, Orian and West New York)
607: USA-new york (Binghamton, Almila, Bath, Norich and South Central new york)
63 1: USA-new york (Suffolk)
646: USA-new york (new york City: Manhattan only; Overlap with 2 12 and 9 17)
7 16: USA-new york (Buffalo, Jamestown, Niagara Falls, Tonawanda and West new york)
7 18: USA-new york (except new york and Manhattan; Coverage 347 and 9 17)
845: USA-new york (Poughkeepsie, Middleton, West Point Military Academy, Newburg and Southeast new york)
9 14: USA-new york (Westchester County)
Baidu Encyclopedia-new york