Even if this is off the poll,I agree with the Nid's.I don't know much about the Flood,but I do know that necromorphs can't operate a spaceship to travel between places,maybe the creator of the marker can,but we're talking about the necromorphs here.
The Nids can,and when they do,they ravage entire systems leaving nothing alive.If the Flood are technically living(and considering that they infect other creatures,they are),then the Nids will consume them.
If a single still mostly human super soldier can stop the Flood for a while,what does that tell you when entire Chapters of truly super human Space Marines can barely do anything to stop the Nids?
But I suppose that if I had to choose between your two choices,I'll go with the necros,because they scare me the most,popping out of vents and whatnot.