I haven't really kept up with this, but last I heard virt-manager was deprecated (on RHEL at least) in favor of just Cockpit, hence the comparison.

Cockpit is definitely nice, but it still feels pretty incomplete compared to virt-manager.

virt-manager appears to still be developed, though, just without the same blessing/level of support from Red Hat.

virt-manager is just a shim on top of libvirt same as cockpit's VM component. So there really isn't any excuse on that front. Although your comparing a tool built as a competitor to something like vmware workstation/virtualbox against a general purpose machine mgmt tool that happens to be able to do some VM mgmt as well.

PS: I think both are good, but I too use virt-manager for all my VM twitting because nothing else on linux is both as feature complete for qemu/KVM while also avoiding having to read the manual just to adjust some VM parameter.