Re: It doesn't matter
If you are hosting Linux/FreeBSD VM's on Hyperv (With Samba 4.0 Domain Controllers on simple single board computers) it will still cost nothing. (And stuff like Live Migration will work just fine.)
Works quite well with the OS on USB.
If cost is an overriding concern you can get away with not having to have a seperate machine in the datacentre for purely admin related stuff which most of Linux options seem to require.
(Xen + Solaris + ZFS would have been the best if it was just bugfixed but it never came to be. Hyperv usb booting + ReFS is about the closest and it works pretty damn well).