The Plextor is a good choice. Given a choice between Internal and External, I would go with an Internal. If you go with the external you should consider the following. USB 2.0 or Firewire. USB 2.0 has a max thru-put of 480 and the Firewire is 400. Do the math.
I have four external hard drives connected via USB 2.0 and they work just fine.
Having said that; I do use firewire. I use it to capture digital video to one of my hard drives thru a Canopus ADVC110 which is a Analog to Digital Converter.