They are both very capable VM environments. I believe VirtualBox has the edge since it will allow you to take "snapshots" (similar to restore points) that will allow you to revert back to a previous point.
You can't go wrong with either one, they're both free so try them both and see which you prefer.
It is a tea versus coffee question. I have used Virtual Box for many years but always had some little gremlins with it. Now I am using VMware Player to run 6 virtual systems (Linux and Windows 8). It is more elegant and easier - I think.
I'm not sure what you consider to be a "supported Guest OSes" but I personally have XP, win7, windows server08, windows server 2012r2, Ubuntu 12, an edian firewall and a dns server (centos 6) running from my home hyper-v server.
Supported Guest OSes are those OSes that are supported by MS and MS will provide support should you have any problems setting them up or running them. Here is the list of support guest OSes for Hyper-V:
Windows 8.1 Windows 7 with Service Pack 1 (SP 1)
Windows Vista with Service Pack 2 (SP2)
Windows XP with Service Pack 3 (SP3)
Windows XP x64 Edition with Service Pack 2 (SP 2)
There are only a couple of versions of Linux supported.
Microsoft supports all their current products so you would need to add at least server 08, 12 and 12r2 to that list nevertheless I think I do see your point… I'd personally feel a little weird asking Microsoft for help setting up my edian firewall but I'd like a surety here, are you saying that a client using virtualbox to install say "QNX 4.25" can ring their helpdesk to sort the issues out?
To be clear, whatever software does the job and feels right 'to you' equals a good fit and more power to you.