You have an incoherent mental threat model of this. Those two systems are functionally identical to the end user to which they are being sold, but one is vulnerable (in this specific way) to the actions of other customers with the same service provider.
Usecases with multiple users on one piece of hardware make no sense? Is there a reason (besides the previous question) you are ignoring the ability to use containers as an alternative to virtualization for all of the perks they provide?
I'm not ignoring containers at all. Linux, FreeBSD, and OpenBSD all have some form of user mode containers. All POSIX systems also have user ids. Linux containers are probably the most functional (no citation here -- I know lots about Linux containers and very little about FreeBSD jails) and are also probably the least secure, because of the aforementioned functionality and because they're rather new. (On the other hand, a really well designed Linux seccomp sandbox is probably the most secure option of all.)
Linux on Xen also allows you to have multiple Linuxes on the same Xen machine. This is the most functional of all and probably also the most secure of all, XSA-108 notwithstanding.
(Also, I find this all rather odd. If you want to compare Linux+Xen to OpenBSD, XSA-105 and XSA-106 much bigger deals. They allow code in a Linux container or other sandbox to break out by exploiting a Xen bug to take control of their Linux host.)
