I have installed both systems and this is what I had to modify on both of them after installing the VirtualBox Additions:
I modified the /etc/X11/xorg.conf file and added this to section Device:
Driver "vboxvideo"
And this to the section Screen:
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1600x1200" "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
