There is no official document released for feature comparison, but there is no functional loss in ECE.
ECE is more streamlined and focused on email and chat. Key new features include New and enhanced agent console which is supported on multiple browsers (IE, Chrome, and Firefox). Also, support for latest and greatest UCCE release. ECE also supports SSO this changes the agent experience and also a better experience and better security. Precision routing is another key feature available.
Hope this helps!