These releases fix several bugs in both versions including several security-related issues including CVE-2014-3668, CVE-2014-3669 and CVE-2014-3670. In the 5.4.34 release there was also a fix put in to correct a regression issue in the OpenSSL functionality.
As both of these contain security-related fixes, it's strongly recommended that you upgrade as soon as possible. As always, you can find the latest downloads on the main downloads page or windows.php.net for the Windows users. The full list of changes in each of the versions can be found in the Changelog.