SolusVM & Virtualizor are popular because they came around the time as a viable alternative when HyperVM collapsed. WHMCS integration modules had since been done to cater to these control panels but I believe most users would still login to the VPS control panel to manage their VMs because the WHMCS modules can only do very rudimentary work.
SolusVM is an ideal choice for virtualization and it is a tried and tested option. Though Virtualizor is around for a few years, scalability still appears to be a problem. A small VPS hosting service, but not a medium or bigger player in the field, is claimed to be a good option. Scalability with SolusVM is not a concern.
However, I do believe that most of my clients would prefer the client end of SolusVM more than Virtualizor, but they are both good panels. I hope that SolusVM (v2 or anything) will soon be done and released, that a lot of people will be satisfied with it, but I agree that either one will be perfect. Otherwise, I would just create my own alternative and use it. I use SolusVM and don't intend to step away – partially, but moving back would actually be quite a problem if it was decided.
We used SolusVM for several years which we never had any issues with. Moved over to Virtualizor recently and there modern UI update beats SolusVM at the moment. Waited for SolusVM v2 for many years for it to never come...
Proxmox is a very popular product because it is free and very well optimized.
As an alternative, I would recommend Hyper-V but it is on Windows and I think you alone understand what it means to keep the infrastructure on this OS.
VMware is a good virtualization system, which is why many people prefer it.