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.
So imagine you have a high attack, but no piercing and the enemy has high defenses. You will not be doing as much damage to the enemy as you would if you had slightly lower attack, but a ton of pierce. Most units have pierce of some kind in their kit, for example, Queen Macherie has high SPR pierce. So you would most likely benefit more from Magic Res pierce for her to round out her kit.
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.