Actually the linked drive will not do both - it’s purely a network device. The USB ports on it are to connect a printer - this drive also acts as a network print server.
Network drives (NAS) are just and only that - they sit still and live out their lives on the network. They also tend to be large, with four or five drives inside the box that can be configured in various RAID setups.
Portable drives have either or both USB / Firewire, and will not have network connections.
Some NAS drives have USB connectors on them in order to plug in a portable drive to backup the NAS drive, but that’s not the same as being able to carry the NAS drive around like a portable.
You’ll really want to buy one of each as there’s no such thing as a good lightweight, easily portable NAS drive yet, and probably won’t be. Drive space has gotten absurly cheap. (I remember the days when there was much rejoicing that the cost had gotten under a buck per meg - now we’re down to 30¢ per gig.)
If you’ve got an iPod with some empty space, you can use that as a portable drive - just keep in mind that iPods aren’t meant for long-term disk use like this and can potentially overheat if the drive’s running for too long.