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.
__________________ WPAuto.io - WordPress Automation Tool for Web Host
Let our WordPress Staging, Cloning, Snapshot Backup, Security & Performance Improvements Software enhance your Managed WordPress Offerings to your customers!
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.