One of the sites that I frequent for accurate fireams information if the Firearms Tactical Institute. As they are aligned with Martin Fackler's International Wound Ballistics organization, they have a bit of bias in their opinions; but not on their tests.
The latest Tactical Brief
from FTI includes some great info about defense loads for shotguns. You can find it at http://www.firearmstactical.com/briefs.htm