CDH 6 is classified as a Major Upgrade. CDH 5 is presently based on Hadoop 2.x where as CDH 6 has moved forward to Hadoop 3.x. There are many feature enhancements and changes across the platform related to enhanced capabilities, performance, and security.
If you desire features only available in newer releases of Hadoop and it's components then CDH 6 may be the version for you though we do intend to maintain CDH 5 in accordance with our EOL. The overall life of CDH 5 is subject to change based on current external activities.
If you are planning a new cluster deployment that does not have any data it may be a good idea to make this decision early. While there is an upgrade path from CDH 5 -> CDH 6 we have restricted those paths to certain releases though we expect to address additional releases as time moves forward.
--- Customer Operations Engineer | Security SME | Cloudera, Inc.