On a Vampire Lord, my answer will be neither, since I prefer Charmed/Enchanted shield over OTS, and won't have points left for PoS after taking Ogre Blade and 4+ ward.
For an SGK, I'd pick OTS, since the built-in regeneration allows you enough points to take Giant Blade over Ogre Blade, and the ONLY reason not to do so, is that you won't be able to fit in a scroll or book on the SGK as well.
If you're running a build without a +Strength weapon, I think Potion of Strength is the better option, since I find that being able to deal with armour/toughness is more important than dealing with ward saves.