Go ahead and download Solr 5.4. The major feature is a new Admin UI which lets you manage Collections, not only cores. The new UI also allows you to manipulate the schema through Schema Browser tab. Also, the Analyzing suggesters can now filter suggestions by a context field, this was one of the issues Cominvent contributed to.
Solr/Lucene version 5.3 is on its way out the door any of these days. In addition to the usual list of bug fixes and optimizations, it adds some new features, mainly related to security. Some are: SOLR-7757: Improved security framework where security components can be edited/reloaded SOLR-7838: An authorizationPlugin interface where the access control rules are stored/managed