420.5n If a permanent has both a +1/+1 counter and a -1/-1 counter on it, N +1/+1 and N -1/-1 counters are removed from it, where N is the smaller of the number of +1/+1 and -1/-1 counters on it.
That persist mechanic video stated that a -1/-1 and a +1/+1 counter cancel each other out and go off the creature but could I not argue , by the rules that, they are both on the creature and get counted?
MTG Rules Advisor
There's no question that needs to wait for Shadowmoor to release to be answered. +1/+1 counters annihilate -1/-1 counters and vice versa. They're not planning to change that. The answer to the question is "No, you cannot use the rules to say that they are both there because the rules say they remove each other."
Counted for what?
The creature's power and toughness after the event? No. After they annihilate each other they are gone.
Whether the creature has lethal damage? Yes. When the -1/-1 counter is placed, the game will check for all SBE conditions. If the -1/-1 counter(s) reduce the creature's toughness to the point where it has sustained lethal damage (or reduces the creature's toughness to zero) the SBE that sends the creature to the graveyard occurs simultaneously with the annihilation of the counters.
Whether the -1/-1 counter will prevent the creature returning? The creature will not return. Persist is (so the Shadowmoor rules primer tells us) a triggered ability that triggers specifically when the Persistent creature leaves play. Thus whether the trigger fires or not will depend on the gamestate immediately before the event. The "event" for this purpose is the SBE check. Therefore the game will see that the persistent creature did have one or more -1/-1 counters when it went to the graveyard, and the Persist ability will not even fire.
There is a bit of an intricacy here that should be clarified. Suppose a 2/2 with persist has a +1/+1 counter on it, some effect gives it -2/-2 until end of turn, and than some effect gives it a -1/-1 counter. Persist will not trigger even though the -1/-1 counter and the +1/+1 counter annihilate each other.
The reason is that both the counter annihilation and moving the creature to the graveyard for having 0 toughness are state-based effects that happen simultaneously. So, at the last moment the creature was in play, it had a +1/+1 counter and a -1/-1 counter on it. Since it had a -1/-1 counter on it, persist doesn't trigger.
(I'm assuming this is because you don't check them in any order you want, you check them down the list)
Merestil: Not true. If the counter is not there, it is not there; if the creature is killed well after a +1/+1 counter and -1/-1 counter remove each other, then it will return. There is not an invisible -1/-1 counter on the creature to stop Persist from triggering; there is no -1/-1 counter at all at this point.