Neither is more likely, since this hypothetical situation can never happen...

But in the spirit of the question I think the nids would just continue to ignore the necrons, unless they had a reason to fight each other... like the C'Tan wanting to eat everything themselves, or the nids attacking them for some reason. But then again the C'Tan might just go back to sleep and wait for life to come back to the galaxy. If they did actually fight, I would say the necrons would win, since the nids will no longer have any more biomass to feed off of, and the necrons wont provide any. The necrons would just beat the nids into submission, being unable to get tired and what not.

But again, this situation would never happen (plus GW would never allow it). Sry nid and cron' fans, the argument has been done to death, neither of you will overcome all the other races. The 40k universe will forever have it's "balance" which makes it such a good game. 8)

 

Number two, the Necrons' orders to exterminate all resistance to their masters' return bring them into direct conflict with the Zer...um...Tyranids

What do you mean to imply by this? It's not like you can't delete a typo, what were you aiming at the with the whole "Zer..." thing?

 

What do you mean to imply by this? It's not like you can't delete a typo, what were you aiming at the with the whole "Zer..." thing?

Blizzard entertainment released StarCraft in 1998. StarCraft is heavily influenced by Warhammer 40,000...

You've got:
Terran(=Imperial Guard in Space Marine armor)
Protoss(=Eldar that are fast, smart, psychic and have actual physical strength. They also play for Old Ones, as they also tutor other alien races, like the Old Ones)
Zerg(=Tyranid clones, they also have "infested" humans that are basically Plague marines)

 

Well, necron's mission is not eradicating every life form, only those who might oppose to C'tan rule. C'tan want to feed out of herds of sentient slave beings.
Tyranids instead eat every living thing they find, leaving behind planets stripped of everything, water, atmosphere, unable to develop life forms again even in a far future.
In short nids are killing the future herds of C'tan , and necrons wont be happy at this.

 

And the Winner

As fans of both i have to say there will be a War between the 2. If the necrons are smart and hide, the ...well, NO the Necrons Can't hide. Why? They are a being. The are made of metal,...The nids can asorb the necrons into there swarm, making a metal nids. Lets say the Nids for some reason the Nids can't detect the Toasters. A fight is un-avoidable. The C'tan Gods that sleep in the tombs where all the attacks are cordinated from, that tomb is where the necron armies are protecting the gods. The Nids will Detect the God without a doubt and when a landing swarm lands and enters the tomb and sees there P!ssed off metal thing walking around shooting Green Sh!t at them the nids are going to say "i don't like what the f#ck is going on here an there going to realize how to dtect the necrons. nids will win.

 

Except that Tyranids use the warp to detect prey, communicate with one another, and so forth.

Because the necrons and C'tan have no warp signature, and have ways of actively reducing the level of warp activity (pylons, pariahs etc) they could just make warp blots on all their planets, and the nids would instinctively avoid them, thinking there was nothing there.

 

So... the Necron would attack first because the 'Nids are killing the cattle... The 'Nids don't get enough "profit", and simply pack their stuff and move to the next galaxy...

Clear as a crystal pylon....

 

I think people are forgetting the supreme might of the C’tan (or at least what they will be capable of upon regaining their full power).

There seems to be nothing in the Tyranid arsenal capable of actually destroying them (note that even the Blackstone Fortresses apparently failed to do the job even if they did wound the Void Dragon).

Lets not forget that the two C’tan we have seen thus far are the weakest by addition (Deceiver) and one that was effectively left to starve for a time longer than recorded history (Nightbringer).

My final point on the C’tan being that they are energy vampires, effectively making the Tyranids look like amateurs at their own game. Tyrainds strip single planets of raw materials while C’tan consume stars and generally lay waste to entire solar systems.

There is another issue with Necron "phasing out" I could see that wasting a lot of Tyraind resources in itself.

 

Not even the C'Tan could destroy the entire hivefleets... They could kill the hivemind... but not all those hivefleets swarming between here and their home galaxy...

...not that they lack the power, but the erea is too big to be properly searched... The hivefleets will simply move to another galaxy if they don't make enough "profit".

 

Blizzard entertainment released StarCraft in 1998. StarCraft is heavily influenced by Warhammer 40,000...

You've got:
Terran(=Imperial Guard in Space Marine armor)
Protoss(=Eldar that are fast, smart, psychic and have actual physical strength. They also play for Old Ones, as they also tutor other alien races, like the Old Ones)
Zerg(=Tyranid clones, they also have "infested" humans that are basically Plague marines)

Yeah, it was basically an ill attempt at a joke on my part. The Zerg bear a remarkable resemblance to the Tyranids and although I do not agree with the usual thesis that "Starcraft is a ripoff of Warhammer 40K", I believe that it was very heavily influenced by it. Except that the Protoss aren't so bloody arrogant. Also, the Terrans were "mere generations away from developing their psychic potential" which seems to imply that they were similar to humans in Warhammer 40K; the Emperor, as I recall, had a plan to safeguard humanity until they could complete their evolution into "Psychic Man".

Back on topic, I just want to make the point that I wasn't trying to incite a debate between Necron/Nid fans and the rest of the community (which hasn't happened, I'm glad) and I know this kind of situation would never happen. I would love to see, however, some different viewpoints on whether such a war could occur and, given the strengths and weaknesses of each race (in a way, they are an anathaema to each other), who would emerge the victor, or would they both survive and go their separate ways?