Both updates promise, in Apple’s words, “improved compatibility, security, and reliability.” The specifics on how the updates do this are unclear, however, as the release notes for both the 10.6 and 10.5 updates are a little light on the details.
Apple does tell us that the 122MB download for users of OS X 10.5.8 and later updates J2SE 5.0 to 1.5.0_24 and Java SE 6 to 1.6.0_20. As with the Java update released last December, J2SE 1.4.2 remains disabled by default, as it’s no longer being updated.
As for the 78MB Java for Mac OS X 10.6 download, it updates Java SE 6 to version 1.6.0_20. It’s aimed at Mac OS X 10.6.3 and later.