In short, plastic bags are probably better — though a reusable cloth bag is best.
And disposable diapers are probably better than cloth, unless you live in an area with a real landfill shortage.
I like their list of 10 big things you can do for the environment:
1. Purchase a more efficient automobile AND drive less
2. Increase energy-efficiency in your household
3. Purchase green electricity and/or generate your own
4. Include more vegetable protein in your diet
5. Teach children about the environment
6. Consider having a small family
7. Contribute money to an environmental organization
8. Give time to an organized environmental cause
9. Live in the city