If you were going to choose between one of the two to go Lightning, I'd go with the Washing Pole. Turning a weapon into Fire or Lightning, it loses it's damage scaling. The Murakumo has good scaling, while the Washing Pole has terrible scaling. You won't get more damage out of the Washing Pole as you level, so it's better to squeeze out it's damage potential through the Lightning build.
The Murakumo, on the other hand, is a fantastic candidate to go +15 with.