Isn't hit magic arrow lower imbue weight than hit fireball? I've never seen it discussed, but it is possible that magic arrow is better damage/second/imbue property weight, if you have say 70 imbue pts left and that is either 33 magic arrow vs 25% fireball. The numbers to be crunched are out there, but I've never seen anyone really look into it. I never use hit spell or hit area when I'm really caring about slayer property, as the slayer damage gets removed from the melee hit when the spell is applied.