Virtulizer gives you a very easy way of handling IP addresses, Proxmox does not. This is for both internal networks and external networks. To me, this alone is worth the price.
Virtulizor also supports both domain and port forwarding, which is what we really need with our kind of set up.
We can create an internal-only network for our Windows machines, and them forward an RDP (Remote Desktop Protocol) port to each machine individually.
So if I have a dedicated machine with 40 Virtual machines on it, I can forward ports 3000 - 3040, one to each machine. Then in my RDP software, connect to any of those outside forwarded ports, and get the machine on the internal network. This can be done with Proxmox, but it's a super pain.