They are both the same. With the "guix system" family of commands you can build containers, VMs, and you can also install a system on bare hardware. Describing it as a package manager really doesn't do the thing justice. You can use all of the "guix system" family of commands even on top of a foreign distribution --- except for "guix system init" which installs the Guix system on your hardware and "guix system reconfigure" which upgrades it on your hardware.