Your projector, or someone else's? If you're going to random business sites and using their projector, it's safer to assume their projector will accept VGA than DVI, and that VGA cabling will be readily available. However, if you're going to do this a lot with other people's projectors, just get both. You don't need to carry both cables -- there will usually be one with the projector anyway.
If it's your own projector, go with DVI.