As a Linux user, why would you want to use VirtualBox or VMWare Workstation? They are not so well integrated with the system, and, frankly, VirtualBox is more of a toy VM player... just go for virt-manager. It gives a conceptually similar interface to VirtualBox, but better integration with the rest of the system. Especially, when it comes to stuff like sending different key combinations.
I honestly cannot think of a single benefit to using VirtualBox (and I'm less familiar with VMWare player) compared to virt-manager. My guess is that it's more often used because it's also a common choice on MS Windows, so, you get more hits if you are going to search the Web for questions associated to VMs / you'd get tutorials for how to set up a VM that use VirtualBox. But, if you apply yourself to learning how either one of these works, you'd see no reason to choose it.