The last comment you say you don't care about sturdiness us a big mistake, remember this a device capable of being moved about. Personally what I have is an old 40 gig hard drive mounted in an enclosure. Heres a link to some enclosures click here and another click here
I would say buy the largest one you can afford as 30/40 gig can be filled up fast these days. Sturdiness is very important with it being moved about. Firewire is good but bare in mind some computers might not have this but more have usb ports. Power supply has just come to mnd aswell. Allthough you can get them powered from the usb port, i find that its own power supply is better. Bare this in mind when you go to another computer to plug it in.