Comparison about Defend and HP stat

In order to increase vitality of a MS, which stat (def or HP) is more preferable? And any differences between 2 of that stat since both of it are increasing vitality of MS ?

For C and B, if you want to increase the longevity, add def

For A and S, HP gains are higher than Def, so go custom HP + OC Def (Because HP has lower gains on OC)