Which one do I download for Windows 7, Monthly Rollup or Security only?
AFAIK the "Security Only" differs from the "Monthly Rollup" in that it only addresses actual security vulnerabilities. The Monthly Rollup has those, but also includes non-security related bug fixes and occasional feature fixes.
Personally I keep my Win7 machines fully patched, so I get the rollups. But if you only want security fixes, then use the other.