If memory doesn't fail me here you lose 1% dmg/hit per level difference. So with 10 levels difference that equals 10%. I don't usually recommend anyone using a weapon until atleast at least 2-3 levels towards reaching the actualy actually requirement cuz missing hits is really a bummer.
Just try and see if it works for you right now and what the difference is! If you fully imbue the sword and couldnt couldn't do that on the level 20 weapon it's probably better to use the bright sword.