>pls add inline if you have knowledge of further points...
>VmWare vSphere/ESXi

ESXI is the free version of VmWare.
It works well, and is crippled, you will want the paid features.
The first paid version "Essentials" is reasonable priced at $560, but the limitations on that version would push my needs into many thousands of dollars very quickly.

If you want to migrate a VM to a different host box using the free ESXI, you really need to have Network Attached Storage.
You cannot easily copy files (or VMs) directly to a new host.

You can use scp to migrate without Network Attached Storage, but the speed is abysmal - crippled apparently by design.
Abysmal = several days offline to copy a large vm to a different host on the local network.

I used FreeNAS for Network Attached Storage, and I found the learning curve steeper than expected.
On the plus side, it works great on old hardware.

Initially I thought the fancy features of the paid version would not be missed.
I was wrong.
Having a large VM offline for hours and hours while cloning/backing-up and all the manual work required has been quite a time suck.

I am seriously looking at XenServer just to get the live migration/backup without going offline.