Just got a Onetouch the 250gb beastie. In the box you get everything you need. Backup software, cables (usb2 and firewire). Nice looking solid silver unit. The drive is available for about £190.
A 250gb internal hard drive is about £150, by the time you factor in the enclosure and backup software (unless you go freeware or you have existing software) its about the same. The speeds tend to be slightly slower using the enclosure. I don't think they look as nice!
At the end of the day much of a muchness. Personal choice.
Assuming your motherboard can take it a pci firewire card is only about £15.
If you are looking its about £100 for a 120gb drive from Simply (using the 10% discount that is available at the moment)
As far as the Onetouch absolutely superb. comes formated as FAT, so I quickly reformatted to NTFS. Very quick. Fits in nicely with the other silverware on my desk.