Proxmox VE is a very fast and powerful solution. It offers feature-rich virtualization, has open-standards compliance, and also includes redundancy and failover capabilities. What I like about Proxmox VE is that it lets you rack and stack two or more nodes and enables you to be up and running with a one-node failure tolerance in very little time. Proxmox VE’s integration with ZFS is also fantastic. It allows you to create pools to store your VM images and data on very easily and their great web UI makes it easy to check drive health, ZFS scrub status, and other things. I think the best part of the web UI is that everything is configurable from the web user interface without having to use the command line. It also has graphs and additional visualizations so you can evaluate the performance of everything. Beyond that, even though you can use Proxmox VE on a single server, the solution makes it easy to set up a high availability cluster on multiple hosts if needed.
Regarding Oracle VM VirtualBox, I would say its most valuable features are its virtualization, its compatibility with older OSes, and its testing of environments without causing interruptions or any harm to production. Besides making it possible to run multiple VMs on a laptop or desktop, its ease of deployment makes the solution appealing. Not only is it easy to set up, but the software is free. Moreover, it has a nice interface. However, I think Oracle VM VirtualBox could use some improvements on its reporting as well as on its network settings for VMs, which can sometimes be hard for the average user to find and understand.
Conclusion: While Oracle is a safe and excellent option when it comes to virtualizing an operating system, I would suggest Proxmox VE because it is newer, has a lot of powerful features, and is a very reliable and stable solution.