I never tried VMF so it may be faster, sure. The thing about VBox is, like I wrote, features. VBox is basically a full-featured Virtualization-Suite, has multiple UIs and an API. It is also available on Win7,8,10/macOS/Linux and Solaris.
You may add: excellent documentation and friendly support (board, irc, list).
If I'll ever have to switch, I'd first need feature-parity.
I was happy with VirtualBox's (on OS X) performance until I bought VMware Fusion. I didn't do any benchmarks so I don't have numbers to back up my claim but it definitely "felt" much faster than VirtualBox.