I choose Tirfing over Khalitz. Dullahans (size medium), as single enemy are the ones that deal the most damage in Niff. While Disguises (size medium) being the most plentiful when you mob. Now, when I mob, I always have a large number of Disguises in it, and usually go after any Dullahan I see. Thus I have quite a lot of Dullahan and Disguises in my mobs. While not being the most serious threat by themselves, a large group of Disguise can do some serious damage. So I would rather reduce the damage I receive from those two monsters in particular, than any others. Hylo and Quves are small, and the damage Quves deal is laughable and Hylos dont deal much damage either (after basic niff gear reduction). Gibbets (size large) are too scarce in Niff to cause any threat while mobbing. So for me its Tirfing>Khalitz in Niff.