These are the reasons that I believe that Knull would win decisively.
Knull is a far bigger threat than Annihilus. Knull has the resources to take over Annihilus' entire armada of roach warriors. Thanos was at the 2nd weakest state in his entire history. Galactus was in no position to fight anything. A&T were written to be fodder. The heroes were cool but far out of their league. That version of Surfer wasn't enough to stand against Knull. The rest of the Heralds would either be brought over to Knull's side or die.
Knull wins.