To add to the confusion... ;-)
If you don't get SSDs for storage but spinning HDDs you want a controller with flash cache and a supercap, to eliminate the "RAID 5&6 write hole" (google it) and also to enable "writeback with BBU" on the controller, which is much faster than write through (hundreds of MBytes/s instead of only dozens). The reason is that any RAID hardware controller will for data safety reasons turn off internal caching of SSD and HDD. For SSD transfer rates and speed is usually good enough to get by without any serious caching, for HDD seek times and command acks delayed until actual write is done will kill any performance. A good RAID controller will realize power is going out, will mirror anything left unwritten in its RAM cache to its onboard flash chips by utilising the super cap battery for energy and when power returns, it will continue writing the data to disks to ensure data consistency.
If this is for learning only you could of course go the very cheap route: Use an internal USB thumb drive to install ESXi on, then get one of those IBM M1015 6 Gbps cards or higher, do not flash to IT mode but stay with original firmware and set all virtual drives in BIOS to "always write back", and also turn on any write cache for all connected drives. This will work with good performance, just make sure you never experience sudden power failure.
That is why for any serious home or data center use you should always buy SSDs with "power loss protection" (i.e. big capacitors in the device) and support the storage server(s) with an UPS to avoid getting hit by power transients.
If you don't get SSDs for storage but spinning HDDs you want a controller with flash cache and a supercap, to eliminate the "RAID 5&6 write hole" (google it) and also to enable "writeback with BBU" on the controller, which is much faster than write through (hundreds of MBytes/s instead of only dozens). The reason is that any RAID hardware controller will for data safety reasons turn off internal caching of SSD and HDD. For SSD transfer rates and speed is usually good enough to get by without any serious caching, for HDD seek times and command acks delayed until actual write is done will kill any performance. A good RAID controller will realize power is going out, will mirror anything left unwritten in its RAM cache to its onboard flash chips by utilising the super cap battery for energy and when power returns, it will continue writing the data to disks to ensure data consistency.
If this is for learning only you could of course go the very cheap route: Use an internal USB thumb drive to install ESXi on, then get one of those IBM M1015 6 Gbps cards or higher, do not flash to IT mode but stay with original firmware and set all virtual drives in BIOS to "always write back", and also turn on any write cache for all connected drives. This will work with good performance, just make sure you never experience sudden power failure.
That is why for any serious home or data center use you should always buy SSDs with "power loss protection" (i.e. big capacitors in the device) and support the storage server(s) with an UPS to avoid getting hit by power transients.