It's pretty funny how similar they are. The Flood spread until they can form a gravemind, and then they're much more motivated and organized. The Necromorphs spread until they've infected a planet to make a brethren moon. I think the fairest situation would be to infect one planet with a sample of Flood spore, and place a marker on another planet. Since the marker is essentially a proto-gravemind, I'd say the necros have an early advantage. However, the Flood spreads significantly faster, and with the ability to absorb the intelligence of the host, I think would be a bigger threat in the long run.