Go with DVD.

1. It's cheaper
2. It's slower, but there isn't going to be a lot of wasted space on any one disc
3. More environmentally sound, since either will likely be "disposable" at some point in time. A disc is plastic and some places can recycle them, there's no solder and PCB and components to deal with.
4. Most people aren't savvy enough to make immediate use of a USB key - while most DVD players will handle picture discs (make sure to save them a set of JPG files if you're also giving them RAWs)

You're keeping an archive of all the work for yourself as well right? Then it's easy enough to make them a new disc in the future if they need one.