If you twisted my arm and made me pick one, I would have to go with upgrading my OS over upgrading Office, that would get very little use.
But if I have it my way, I won't be putting Vista on a PC I own till MS twists my arm in about 5-6 years.
Hopefully by then there will be a real Windows alternative, because for me Linux is NOT an option...I'd sooner go back to Win98SE.