Welcome to DakkaDakka
Well being a Necron collector myself, I shan't be one sided, Orks do have good looking models and such making them a nice looking mob army, But if wanting to play with a army of either Orks or Necrons...
Do give warning that Orks have lost the Mob special rule in 7th making them really have no use, but Necrons are less fielding but they have chance on 5+ to stand back up when wounded and killed (unless its instant death).
But end of day, your choice but they both just as good in visual wise, on table top matches, Necrons are winning over Orks in playable.
Hope you enjoy your time here