Both SharePoint 2016 and Project Server 2016 Release Candidate are not available. SharePoint 2016 RC is mostly feature complete and it’s now safe enough to evaluate it to get an overall picture of the final product. If you have older Beta 2 then it can be upgraded to RC.
If you have InfoPath 2013 and SharePoint Designer 2013 then they will continue to work with SharePoint 2016. Both will be supported for the duration of SharePoint 2016’s support lifecycle.
You can read about the prominent changes made to 2016 version in this blog post: