This is one of those questions I hate, because it seems there's no right answer. I still remember going to the grocery store as a kid with my mother, and all the bags were paper. Then plastic ones came out, and everybody said 'plastic saves trees', so people started getting plastic. Then people said 'plastic is not bio degradable, it gets hooked on dolphins' noses, etc etc. So, back to paper. Now I'm a tree killer again... as for those reusable bags, the store in my area sells them, but they're pricy, close to $10, I think there's some kind of deposit system to lower the (eventual) cost, but the up-front made me say to hell with it.
It all reminds me of the 'great egg debate', as I like to call it. I remember when eggs were good for you, and many vegetarians talked about what a great protein substitute they were for meat. Then eggs were found to have cholesterol, and you shouldn't eat them. Then scientists discovered there was good and bad cholesterol, and eggs had the good kind, so eat up!
I think of these things whenever the global warming arguements make the rounds, the sorts of arguements and flip-flopping on what is good or bad are similar, in my mind. The number one conclusion I come to in all this is that recyling and 'being green' is for well-to-do people with 'prosperous guilt'. Poor folks just do what is cheapest, perhaps with a mental pang or shrug, perhaps not.