I've worked at a health food grocery store for about a year as a bagger. It boggles my mind when I think about how many times I ask whether people want paper or plastic bags. What boggles me even more is how people react.

Some people come and want triple plastic.
Some want double paper.
Some are convinced that items bagged individually are the only way to get them home.
Some want plastic bags IN paper.
Some want paper bags IN plastic. 
And some get offended that I would even offer a bag because they already have one or can handle the inconvenience of juggling a few items 25 yards to his/her car. 

In a health food store, one would expect most to choose the last option scenario. 
Paper or plastic? To choose one and waste resources, or choose neither and help the environment? 
Yes, the most responsible option is to bring your own canvas/hemp/re-used grocery bag, but sometimes you make an unexpected run to the store, you forgot your bags, etc., and must choose the lesser of the two evils. So, which is it?

Paper AND plastic are materials that downcycle when recycled (meaning they have a finite life and loose quality with every recycle). 
Paper biodegrades quicker than plastic. 
Plastic uses less energy to make.
Paper bags are more durable to reuse for grocery shopping.
Plastic is easier to reuse for trash bags.
Paper bags are already recycled.
Plastic bags save trees. 

They each have their pros and cons, but, ultimately, they are both evil. Do you save trees or energy? Both are equally important and go hand in hand. If you MUST choose one at the store, think about what you can reuse most in your home. They each help the environment in one way and degrade it in another. The investment for a reusable bag is not much. JUST ONE BAG CAN MAKE A bigger DIFFERENCE than you think.