>>16 a HD doesn't even come close to maxing out the transfer rate of either specification.
Correct. True of IDE interface, even. (UDMA100/133/150)
But I hope you're not trying to imply that your friends' choice of interface has any bearing on the failure of their drives, rather than the brands they chose. Firewire consistently performs better than USB2.0 in real-world performance, but that doesn't mean it'll keep your drive from dying. Their drives have failed because Maxtor fails and WD sucks (to a lesser degree).