I would choose apoc 100% because knull needs dupe and high sig for his full potential whereas apoc doesn't, to get the fullest out of apoc all he needs is 4 charges (just use cable synergy to start the quests with 4 charges) all apocs dupe does is give him health when you don't use him for a few fights so it's not needed.
Tough choice, I personally think Knull is the more fun and damage dealing champ whereas Apoc has fantastic utility. If I already had big damage/good cosmics like Herc, Galan, etc. I'd go for Apoc. If you feel like you're lacking in the cosmic champs department I'd go Knull But in either way Apoc is probably the better choice
(I'm just a really big Symbiote/Knull fan in the comics)
It depends on what content you have left to finish. If you still have loads of story content, Carina's etc still unfinished, Apoc is without a doubt the best option. However, if you're someone who's already done with everything and is focussing more on Battlegrounds, Knull is your guy.
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If you feel like you're lacking in the cosmic champs department I'd go Knull
But in either way Apoc is probably the better choice
(I'm just a really big Symbiote/Knull fan in the comics)