The 2018 version of Start Collecting Necrons has Necron Warriors (plus Scarab Swarm), the same Necron Overlord, an Annihilation Barge/ Catacomb Command Barge and a unit of Necron Immortals/ Deathmarks.
If you were to buy these separately at retail you'd be paying $194 AUD.
So monetarily 2018 wins, and with two combi-kits in the 2018 version you also have more flexibility and more left over bitz (inlcuding the Necron Overlord left over from the Catacomb Command Barge if you build the Annihilation Barge option).
Each of these sets give a really nice core to your army, but the 2018 version, as well as being more flexible, gives you more options in terms of tactical play with the Deathmarks/ Immortals combi-kit giving you either snipers or heavy hitting power, plus the Barge option mentioned earlier.
The 2016 version does have the Triarch Stalker which is hands down one of my favourite models EVER, but the room it takes up in the box means you miss out on some other goodies - which are in the 2018 version.
Long time Necron players, let us know if we've missed something here. Always happy for feedback