You get the odd few that support both, but I tend to stick with Western Digital since they've never let me down before.
The Western Digital Studio range have Firewire ports as well as USB.
The smaller drives with no external power are the 'My Passport' drives.
The larger desktop HDD sized ones are the 'My Book' range.
Either way, they always come with all the relevant USB and Firewire cables etc in the box as well.