LION:
Safari 5.1.5 contains a fix for an issue that could affect website usability when running Safari in 32-bit mode.

SNOW LEOPARD:
Safari 5.1.5 contains improvements to usability and stability, including changes that:

Fix an issue that could affect website usability when running Safari in 32-bit mode
Fix an issue that could affect the stability of Software Update on Mac OS X Server v10.6.8