Hi Andreas, I’m inexperienced in your cellular model but If I were you, I’d utilize NAC for two reasons:
1)this small molecule itself scavangers free-radicals particularly the thiol (sulfhydryl) group confers antioxidant effects and is able to neutralize free radicals.
2)Acetylcysteine is the N-acetyl derivative of the amino acid L-cysteine and it is one of the indispensable amino acids that constitute glutathione so it could contribute to increase the cellular glutathione pool, that is the most important antioxidant molecule inside the cell.
So you could obtain more powerful effect than idebenone..but it's just my opinion.
Definitely, NAC is more potent than Idebenone, because the later acts just like glutathione (considering the differences in pharmacokinetic properties); however, there is no general idea regarding which compound is more potent in this respect, because there are many factors play a role including the model that you work with.