I never saw the benefit of going for horn of valor over defensive stats. I'd rather get the three passives in the SnS tree. Not getting flanked mean you can rely on the AI to tank, less damage from archers mean you won't get absolutely murdered from afar and the 20% less damage seem to be calculated before anything else, meaning it's absolutely huge.
Guard, on the other hand, is just a flat out "shield". A full guard is 25% of your max health. Assuming you rock 1k health (That's late game, by the way), you got an aditional 250 at full guard. That's not a lot. Without any defensive, that's a single arrow from a low level enemy. You need defense to capitalize on it.
The other problem of going right for Horn of valor is the very first levels. Now, in a normal playthrough it might not matter much; but if you play with all trials on, on Nightmare, you won't get to level 4 before you are 1/3 done with Hinterlands, which includes roughly 4 rifts and a whole bunch of fighting. This means you will not have a taunt for roughly 1-2 hours of gameplay, against enemies that will murder anybody but the tank, and your tank will slowly die a painful death, since you lack any sort of solid healing that early and the tank can't hit fast enough to generate enough guard to not lose health.
If you really feel like the Horn of Valor is going to help you (The bonuses are rather meh pre-skyhold, without any sort of way to capitalize on it), then you could consider dropping Turn the Blade. With that in mind, I have yet to find anything stronger than Turn the Blade, Trust the Steel and the Masterwork Vanguard Armor found in Hinterlands (http://dragonage.wik..._Vanguard_Armor). After that, your tank will most likely never chug a potions unless bombarded by wraiths and/or archers or getting mauled by a 2 handed mauler.
Now, I will admit Horn of Valor is extremely useful. It allows the tank to top of his/her guard, which barely goes down with enough defense, it allows the squishy party members to survive one extra hit(Which is very important with trials!) and it allows any fast hitting character to tank any low dps enemies during the duration(Such as a mage or a DW rogue), which is also very useful. So, yeah. I can see the benefit of it at level 4, but I still think(and feel) it's inferior to running full defensives with taunt on the tank.