History shows that there will come a day that Oracle (owner of Virtualbox) will shut down the project or commercialize/licenses it. So best to choose a disk format for your VM's that can be 'converted' to other virtualization platforms. (MySQL, OpenOffice, Open Solaris, Java, ring any bells...? 
Yeah it was my big nightmare few months ago, that oracle can shut-down the project, but i feel that they have learnt about the open-office project. You can see now the leader is Libreoffice, Openoffice is only a shadow of what he was before oracle comes.
I think that oracle want to be considered as a sponsor, but i do not know for how long it can pay.
When you look about linux distro as debian, this software is a part of their "free repository", i hope if they do this, we can believe in a good future.
But stay suspicious

For now by default you can choose
VDI (VirtualBox Disk Image)
VMDK (Virtual Machine Disk)
VHD (Virtual Hard Disk)
HDD (Parallels Hard Disk)
QED (QEMU enhanced disk)
QCOW (QEMU Copy-On-Write)