We are running SolusVM and we are currently testing ProxMox for some features we are after.
Proxmox is an excellent solution quite versatile packed with a lot of features. KVM runs very well and deliver excellent performances.
I have used proxmox for a few years now and its been really stable. No issues yet.
We have tested the cluster features within proxmox which is a really cool feature.
But I would choose proxmox for more advanced users and virtualizor for less advanced users.
I think virtualizor is easy and all the popular billing panels come with the plugin to deploy vps using virtualizor. So its made for vps sellers as first choice users. Whereas the Proxmox is a little complicated and requires a learning curve to set it up.