You can find the answer at the Institute for Lifecycle Environmental Assessment website.
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 less2. Increase energy-efficiency in your household3. Purchase green electricity and/or generate your own4. Include more vegetable protein in your diet5. Teach children about the environment6. Consider having a small family7. Contribute money to an environmental organization8. Give time to an organized environmental cause9. Live in the city10. Vote