What's the difference between JDK 8u91 and 8u92?

The Oracle website is clear: use 8u91 (the 'CPU' release) under all circumstances except where you specifically require fixes present in the 'PSU' release (8u92, in this case). The difference between CPU and PSU releases is explained via a link on the release page.

For future reference, the exact quote from the Oracle release blog is:

JDK 8u91 and 8u92, two new Java 8 updates are now available. Oracle strongly recommends that most Java SE users upgrade to the latest Java 8u91 CPU release, which includes important security fixes. Java SE 8u92 is a patch-set update, including all of Java 8u91 plus additional features. You can download the latest JDK releases from Java SE Downloads page...

It's a matter of perspective. What Derek didn't quote was:

Java SE Patch Set Updates (PSU) (i.e. u92) contain all of fixes in the corresponding CPU (i.e. u91), as well as additional non-critical fixes. Java PSU releases should only be used if you are being impacted by one of the additional bugs fixed in that version.

For that you need to read the Release Notes and see if you need the additional fixes.

JDK 8u91 and 8u92, two new Java 8 updates are now available. Java SE 8u92 is a patch-set update, including all of Java 8u91 plus additional features. Please check the release notes below for JDK 8u91 and 8u92