While I understand what the stats seem to do in function, the whole damage calculation system in this game is very poorly explained. I've noticed that newer equipment tends towards piercing, which means I assume that its the better option, but can someone confirm this to be true? Same with Mag Up VS. Spirit Piercing.
PvP content piercing tends to be better. PvE attack tends to be better.
Depends on who you are attacking. If you attack a mage who has low defense, the ATK UP stat is better, however, if you attack a tank, the DEF Piercing stat is better. See what you will face the most or what you want to kill. Same with Mag Up VS. Spirit Piercing.