Even if Anderson has the Nail, Kotomine's black keys can negate regeneration. Anyways, Anderson has the advantage of firepower while Kirei has the advantage of skill and physical strength. In his last bout against Kiritsugu his punches were as strong as a grenade explosion. By burning up 10 of his Command Spells at once Kirei can also temporally gain the stats of a Servant. Even without the Command Spell boost, Kirei can easily disappear from sight and deflect supersonic daggers. With slight boost he became a supersonic+ combatant himself and bitchslapped rifle bullets. In terms of ranged attacks, Anderson's bayonets and Kotomine's black keys are roughly the same. They can both throw them with inhuman accuracy and en-masse. Although Anderson can spam more bayonets and for longer than Kirei, thanks to his 4th dimensional skill. On the other hand Kirei can cast illusions to confuse the enemy and ensure that his black keys would hit for certain. Unfortunately, Kirei can't pin opponents by their shadow. Neither use the sacraments like Ciel did. I say this is a pretty close fight but Kirei may have a slight edge.