Depends on the level. If you have the scabbard of keen edge it is likely that you are atleast 5th level so I will base my assumptions on that.
GIven a choice, I would go with:
Weapon: For a +1 bonus I'd go with o one of the energy bonuses (fore, frost, shocking, etc.) that give the extra d6. Holy is a good choice too but that is +2. Speed is the best high level choice at +4 but it is reproducable (spell, boots, etc.)
Armor: I like the resist element option here. Others probably have better ideas!