This ones actually kinda interesting. On one hand, the Gravemind is inert currently in the series (since he technically isn't /dead/ until flood have been eliminated as a speices(?)) The Necro's happen to still have one or more Brother Moons active and alive. And from my understanding, Necromorph host bodies more durable than those of the Flood. Ultimately I think that Necromorphs would outlast the Flood in a fight provided that we're talking about the current state of each universe. If we level the field, giving the Flood a Gravemind and the Necromorphs a Brother Moon it could be anybody's game. But if we have a pre-ring detonation flood versus the Necros, I have no doubt that the Necromorphs would be hung out to dry.

They'll just infect each other..until the universe is dead.

The flood have hive minds (gravemind). Each person consuned by the flood has their knowledge and memories taken. It can then be used for every fllood form Tell me how you can get any more OP Oh and the flood has the logic plauge to essentially highjack every forerunner AI it touches

The Xeelee collapse the galaxy and then bugger off.

If I remember correctly, the necromorphs require two things to create a necromorph when infecting an area: 1. A corpse with enough limbs 2. A type of necromorph capable of converting the corpse Aside from that they don't really have a means to control technology for the most part, but possibly enough understanding to disable it. It also does not help that (even if this is a case of a difference between gameplay and story), necromorphs aren't very organized outside of the matter of providing the corpses and means to cause a convergence. Whilst a depatable point from a debatable game in the series, necromorphs seems to be rather single-minded on the corpse portion of the mission, and not enough on the defense of their markers. They also lack some amounts of coordinated attacks. Now, the Flood can infect living (and possibly dead) life forms, provided they're sentient. Controlled by a greater mass such as a gravemind or maybe even a keymind (provided my memory is correct and those do in fact have a chance to exist in the Halo universe and aren't just something made up), their hive mind seems to have a greater organization, allowing the simplest spore to be obedient to the point of being able to ignore potentially infectable targets for the sake of a bigger target. The fact that the flood-infected individuals can also operate weaponry from their previous owners, control ships (to a degree), and handle AI's with their logic plague, they have a technological advantage alongside a wider infection range. Issues of durability could become an issue if it turns out that a spore can't take over a necromorph, but that is a question we'd probably need a writer from both sides to agree upon (same going for how a marker would affect the minds of flood nearby, and if they can shape the greater mind leading them or not). So to make a long story short: my vote is flood for now, but I may need some corrections in my arguments here and there.

Correct me if I'm wrong, but doesn't the Marker infect all life? So, technically, all the necromorphs need is a marker nearby to infect the flood.

Explain your decision