external drives just are internal ones housed in external boxes. if u have firewire ports, get a firewire external drive because firewire is still faster than USB 2. however, if u want to bring your HDD to different places that may or may not have firewire, but probably have USB, then get an HDD w/ a USB interface. or even better, get one w/ dual firewire and USB interface.
alternatively, u could buy your own internal HDD and install it yourself in an external box - very easy. you can also get laptop boxes to put such drives in - the boxes are much cheaper (but the drives aren't) and they're much more portable. some can even be powered solely off the bus, so a power adapter isn't needed.
some external drives are stackable or have special, i.e. backup, features not found on other drives. u probably wouldn't need these features - they're generally corporate/business oriented. maxtor makes such drives.
there aren't really any brands to avoid. decisions should be based on price, interface, performance, mobility, and aesthetics.
