Neither.According to my copy of the Handy Science Answer Book compiled by the Science and Technology Department of the Carnegie Library of Pittsburgh - third edition - the answer is; neither.
Both are environmentally harmful and the question of which is the more damaging has no clear cut answer. On one hand, plastic bags degrade slowly in landfills and can harm wildlife if swallowed. Producing the them pollutes the environment. On the other hand, producing the brown paper bags used in most supermarket uses trees and pollutes the air and water. Overall, white or clear polyethylene bags requires less energy for manufacture and cause less damage to the environment than do paper bags not made from recycled paper. Save and reuse any paper or plastic bags you get (which I do both) and when possible or plausible, bring your own reusable bags or containers to the store.
