It boils down to this: use shield oath for trash. Don't bother using sword oath because they'll be dead before you know it. I would actually advise to learn to pull bosses in shield oath and switch to sword oath after you've established some threat. You don't really need to to clear any content but if you so happen to join a static later on you will be expected to tank in sword oath. Yes you take a bit more damage but a healer should be able to heal you up just fine.
You need to look at shield oath as another tank cool down that you can use whenever you want to (if you have the mana) If a dps takes a bit to much damage because he failed to dodge a mechanic or if your healer is getting low on mana or something switching to shield oath might help relieve some stress from the healer. If you're not sure if you'll survive a mechanic, pop shield oath as an extra defense