I'd suggest getting the 'mini' external USB drives; they use laptop hard
drives. The key advantages are USB powered (really nice!), take less
space, really are portable. They're small enough that your son can toss
it in his backpack to carry around if needed.
If he only uses it to back up, reliability probably won't be much of a
concern as it will only be used/powered when he plugs it in to do the
backup.
Performance-wise, they will all be essentially the same as the USB
interface is the bottleneck. You'll see 37-40MBps tops. Firewire,
Ethernet, or eSATA would be faster but for real-world usage it won't
matter.
Likewise, I'm not sure there's enough of a difference between brands any
more to recommend Maxtor over Seagate or Western Digital. Basically, if
you have a preferred brand, get that. Otherwise get whatever's on sale
at Fry's or Newegg. A casual glance shows 250GB for about $100 from WD
& SimpleTech.
I'd also suggest 'real' backup software v. whatever might be bundled.
Something like Acronis TrueImage (
http://www.acronis.com/homecomputing/
).
midrange.com
