PLD seems to be the recommended beginner's tank, but you can pick WAR if you really want to. The main reason why being that PLD has an enmity AOE that doesn't use TP, many straightforward mitigation CDs, stances dont affect how the job is played too much, and PLD's gauge does not have a major impact outside of tankbusters. WAR, while it does also have many CDs, are not quite as easy to use. Raw Intuition will make you take more damage if used incorrectly, and Inner Beast is tied to the Beast Gauge, which itself is quite harder to manage than PLD's Oath Gauge. Plus WAR's enmity AOE, Overpower, is a conal AOE instead of Flash's simple circular area, and can drain your TP if you use it too much. Not to mention Defiance (tank stance) and Deliverance (dps stance) have much larger and pronounced changes than Sword Oath and Shield Oath. All in all I would pick the job that seems the most appealing to you, but if you really are concerned with how difficult tanking could be I would choose PLD.