I don't know how usable SpectrumOS is so far – I guess we'd have to compile it ourselves in order to find out. Either way, it is being developed quite actively, see https://spectrum-os.org/git/

As for how it compares to Qubes, I don't think I'll be able to tell you more than https://spectrum-os.org/design.html & friends. I suppose the upshot is:

- KVM instead of Xen

- One VM per application

- Single file system for user data (to which users can grant VMs access on a folder-by-folder basis)

- Package system from NixOS (nixpkgs) for reproducibility & immutability