ReFS cluster size with Veeam Backup & Replication: 64KB or 4KB?

There has been a lot of discussions about ReFS 3.1 after Veeam released its version 9.5 with support for the block clone API. With this integration between the two product, users can now design a repository that combines the speed of a non-deduplicated array, with some important space savings that usually belongs to those dedicated appliances. We have seen many many discussions in our Veeam forums, and I also published two articles on this topic you may want to read: Windows 2016 and Storage Spaces as a Veeam backup repository and An example for a Veeam backup repository using Windows 2016.
Now that people are starting to use ReFS, another question has risen: which cluster size should I use? 64KB or 4KB?