If a creature with persist is sac'd and is given undying would the creature come back and the counters cancel eachother out? I'm building a {mikaeus the unhallowed} EDH deck and trying to do some research
The trick is to use one, then the other.
As the effect from both is to return a creature from the graveyard to the battlefield with a counter on it, it won't be there for the second effect to take place.
So I choose which effect takes place undying or persist sac the choose other effect?
Yep! It's a never ending cycle if you have a good sac outlet.
Undying first time, persist second, undying third, etc.