Here is a goal: How can I protect trees and bushes from being damaged by weed whackers?

Which way makes more sense to accomplish this goal?

Available choices:
a). Cut the top and bottom off a clear small pill bottle, then slice this cylinder down the middle. It will curl around the tree, stay in place, and be almost invisible.
b). Cut the top and bottom off a clear 2-liter soda bottle, then slice this cylinder down the middle. It will curl around the tree, stay in place, and be almost invisible.
b).