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Currently I have 31 necron warriors with 8 canoptek scarabs, Vargard Obyron, Trazyn the Infinite 5 immortals with tesla carbines and now I can't decide either to buil DOOMSDAY or GHOST ark. Doomsday ark has this awesome doomsday cannon, but ghost ark can repair fallen necrons. So what do you suggest??

Wouldn't this be more of a tactics question than a army list question?

Regardless, Ghost Ark, hands down.
The Doomsday is too fickle to use unless it has a LOT of support (if it moves, the weapon is suffers, but if it stays still, it gets no jink save/is auto hit in CC. Plus, the pie plate is prone to scattering, so kind of needs a Stalker to be most effective).
Really, Doomsday Arks should only be considered at high point level games.

Strategically, I've gotten a lot more use out of the GA but if you use magnets and mount the cannon upside down then you can choose as the situation demands.

Just magnetise it.

Which one is better really depends on who you're playing, how many points you're using and what the rest of your army looks like.

In larger games generally prefer to use the Doomsday Ark because don't usually use huge blobs of warriors and because Night Scythes are generally better as far as transport ability is concerned.