Regardless of the persons home setup, the point is that if you need to access the media from the drive it is accessible to more computers if the interface is USB. Windows computers can access it as well as macs. This increases the user base significantly. If for some reason he wanted to transfer files or media to another computer there would be no hardware compatibility issues. (relatively speaking of course)
There are almost no proprietary file formats that require a specific mac formated drive. Document files, picture files, music files, and almost all other data files can be accessed from a windows applications and apple applications.
As such, USB, being a more universal interface, is a better backup solution in my opinion. Granted firewire is much faster, I would only suggest that interface if the person had access to other computers with a firewire port. If you only have one mac and every other computer you have access to is a p.c. then I would highly suggest using USB. if you have access to other macs with firewire or even p.c's with firewire, then I agree that firewire is the way to go. I simply wanted to let it be known that firewire is not as universal as USB. As such, USB is better for simple backup needs.