Since I've started playing it online I've actually wondered this more..Anyway to me I think both some like Increasing Evasiveness with Double Team/Minimize or Decreasing to oppositions Accuracy with Sand Attack seems to takes luck more then anything aside from Critical Hits because sometimes you use it a lot and the opponent might not touch you or you use it and it still hits (unless it's an attack that never misses) like Faint Attack. But for me I've always wondered what if this Pokemon attacked itself in confusion opposed to hitting mine would that have changed the battle?..For me I think both play a big part in the game..however in some battles one might outweigh the other it seems.

Though I would say mostly skills.

It honestly depends on the person and how they play