"x86 virtualization is about basically placing another nearly full kernel, full of new bugs, on top of a nasty x86 architecture which barely has correct page protection. Then running your operating system on the other side of this brand new pile of shit.
You are absolutely deluded, if not stupid, if you think that a worldwide collection of software engineers who can't write operating systems or applications without security holes, can then turn around and suddenly write virtualization layers without security holes."
For a very long time, Theo subscribed to the philosophy that the way to get a secure OS was to keep it as simple as POSIX and historical BSD would allow him to (and no simpler) while eradicating all the bugs. Eradicating bugs is obviously a good thing, but the track record of that strategy in the real world has not been great.
That's obviously changed over the last 5 years or so, but you should be careful reflecting DeRaadt cynicism from a decade ago into modern discussions.
Qubes is surely a better bet than vanilla OpenBSD.
Is there a concrete reason you believe that or just a gut feeling?
It's pretty obvious to anyone really. Even if you assume that OpenBSD had 0 bugs, it doesn't protect you if someone exploits your browser, or some application that you have, which may have a lot of bugs. In contrast, tou can create two isolated OpenBSD VMs in Qubes, one where you do your banking related activities (and setup the firewall to only allow connections to your bank for example), and the other where you do your browsing, so that even if someone pwns your browser in the second VM they won't be able to steal your banking logging credentials - unless they have a Xen exploit of course.