Security differences between using Monero on PureOS and QubesOS

PureOS is based on Debian, Qubes is mostly based on Xen (a hypervisor). So when you install Qubes you are essentially installing a custom Xen version with added goodies. PureOS on the other hand is just a plain operating system like what you are used to. Xen is what provides the isolation between the different Qubes. I haven't tried it, but most likely you can even run PureOS inside Qubes if that is something you are willing to do.

Also note that you can use PureOS and couple it with a PureBoot bundle as an added security feature. PureBoot provides you a separate usb key so that you can easily verify if your device software has been tampered with while you were not attending it, or during transit from Purism to you.

You can also use that as a second form of authentication to decrypt your disk, so even if your laptop is stolen no one will be able to get access to the files.

More info here: https://docs.puri.sm/PureBoot.html

In general, QubesOS offers greater security than PureOS. With QubesOS, programs are run in their own virtualized environment. This could offer greater protection if the computer was infected by a malicious program. Here's a good summary comment.

Note that you can install QubesOS on a Librem device with any USB. You do not necessarily need to buy the USB stick from Librem directly.