Previously, I got a SanDisk micro cruzer 512MB with Avast! Antivirus U3 Edition. Yesterday, I bought a SanDisk micro cruzer 2GB with McAfee VirusScan USB.
Since these are the only two companies offering antivirus software for U3, it is a good opportunity for me to do a comparison.
Both come with 1 month trial license and offer real-time protection for U3 devices only and not the host computer where they are connected. So, you still need to install antivirus software for real-time protection of the host computer.
Avast allows you to scan U3 device, host computer or a folder on-demand whereas McAfee has the option to scan U3 device only and can detect antivirus software on the host computer. But when you insert the U3 device, both can scan active processes running on the host computer. Avast shows an icon running on the system tray whereas McAfee does not. You need to register with McAfee before the license can be activated. On the other hand, there is no registration requirement for Avast. Overall, Avast is recommended.