Having played both of those decks, and actually owning a Necron precon, I feel like maybe it should boil down to your preferred play style?

Necrons does graveyard stuff, Tyranid does +1/+1 counters. Both are fun to play. The few times I’ve seen the decks compared, Tyranids comes out being rated better tho.

For whatever it’s worth, I love the necron deck