Neither. The Washington Post has a great graphic today comparing paper and plastic grocery bags, coming to the conclusion that neither is a decisively better choice for the environment. Instead, you should choose reusable grocery bags. Environment.About.com recently came to the same conclusion. It makes you wonder, considering how we're nickel and dimed for everything else these days, why are paper and plastic bags still handed out for free?
The Green Miles recently wrote about how he's fighting through the frustrations of the transition to reusables.