Neither! I love the bags Mer recently sent me. The company that manufactures them has a wonderful website, where you can learn more about the harmful effects of traditional paper and plastic grocery sacks. If you're looking for an easy and inexpensive way to make a difference, this is a great place to start, and I recommend this product specifically.
It took me awhile to get in the habit of remembering to bring my cloth bags, but now when I'm done emptying them, I return them to the trunk of my car right away. That works for me! What works for others? Click here for more tips.