virt-manager only requires access to the libvirtd socket, as long as the flatpak.has that as default configuration (which I imagine would be the case), there's zero difference beteween flatpak and native.
The virt-manager flatpak doesn’t work out of the box, you need to do some setup on the host. At that point you may as well use the deb of virt-manager.
that is a good point... I obviously missed that. my generally would only use flatpak from the same developer of the app, or I will just use the deb packaged by my distro.
If you install virt-manager on Debian via apt it will have full system acres and also automatically install and configure libvirt, so this method is preferred.
The standard session requires root, and for some reason this means that VMs couls harm your system more or something
VMs don't have access to the host, so even if the virtual machine emulator Qemu and libvirt require root access, the encapsulated guest virtual machine have no access to the host. They can't harm your system.
Yup VMs dont get access to the system. Unless there is a vulnerability.
For doing malware testing etc, qemu user sessions might be preferred.
You can just use RPM/DEB virt-manager and switch to the QEMU user session anyways. If you dont need some advanced stuff like GPU passthrough (I guess) (USB works) you can use that full time. I do.
Depends on what distro you're on? You say the deb version is 4.0 and flatpak is 5.0, suggesting you may be on a long release distro?
I'd favour the Deb version as it's official for your distro. The flatpak version is unverified; it's extremely unlikely Virt-Manager is compromised or will cause any issues but virtual machines do have security risks.
Also problem solving issues with the flatpak version may be more difficult as you have a whole layer potential issues in the sandbox on top of all the other issues people can have around KVM/QEMU. But you could install it, if it works great, if not, revert to the Deb version.