In my limited experience, probably the CD. Actually, whether it's CD or CD-R is the same assuming they are both readable by a conventional CD player (i.e. the CD-R doesn't contain MP3's, etc that might pose a problem). Most exhibitors still seem to have a CD player or transport. Some may be more reluctant to connect a USB drive because of concerns about viruses (for Windows PCs) - or they should be.
Really all exhibitors (and dealers) should have an i-device dock with digital outs for customers, but few if any do. Yeah, that may not be the ideal transport vs a high end server or dedicated computer, but it sure would make it easier for the drop in customer to demo gear with their music without additional prior preparation. But perhaps that the point : to dissuade more audience / customer demos and / or control the environment for optimal playback quality.