Host Cache is essentially swapping to SSD. More on that here and here. Put enough memory in the host, and you won't have any use for it.
Virtual Flash, called vFlash Read Cache is a host-based flash cache that can be broken up into chunks and assigned to virtual machines which will use said flash to cache frequently uses blocks rather than going to the SAN to get the data. More on that here and here. vFlash Read Cache requires an Enterprise Plus license.
In a VDI deployment - if you're using linked clones, you'd want to accelerate the replica VMs. Unfortunately, you can not use a separate replica datastore if the datastore is to reside on local disk.