VirualBox supports 3D hardware acceleration for OpenGL apps.
I'm using a legacy version of Autodesk Inventor design software (v7) that I could not get to run under Win7. I am testing this on a TX2510 tablet pc with Win7 Ultimate host and AMD-V enabled.
In my experience, XP Mode in Win7, while it will allow Inventor to run, cannot compare to VirtualBox.
Under VBox 3.0 with guest XP Pro SP2, Inventor runs nearly flawlessly with excellent 3D graphics and nearly no lag - despite using an integrated graphics card.
VBox has seamless mode which allows Xp's windows to coexist with Win7. While XP Mode's integration with the host is handy - it's totally useless if I can't run the legacy app.
Both XP mode and VBox integrate my tablet pc's pen and touch screen capabilities very well into the XP environemt.
It's disappointing to find the XP mode implementation so weak compared to an opensource solution freely obatainable.
Go with VirtualBox.