Tanks are the most easy role to play now, enmity is a joke, and gameplay in general to braindead, WAR is more easy compared to PLD at max level but consider the skill ceiling of all tanks it's pretty low right now, so pick one and then the other, if you have no intention to raid or do anything serious with them then just leveling one and then the other at your free time for fun since it won't take much time specially with dungeons being fast for the role, the adventage of being able to level up all jobs with 1 char is for that and stay with the one you enjoy the most at the end.