by Trevin
I’m just going by the wiki, but it says for the Holy Avenger:You gain a +1 bonus to attack rolls against adjacent Monsters while this item is in play. Increase the bonus [BGCOLOR=#FFFF99]to +3[/BGCOLOR] against Undead Monsters.
so your bonus (if you use that instead of the Magic Sword) is only +3, not +4.
Based on the description of these two items, I see no reason to ever use the Magic Sword instead of the Holy Avenger.