The USB2 drive has the advantage of being most versatile. It can go from 'puter to 'puter and should be able to go on a USB 1.0 device if you have to (but probably won't burn faster than 4x). It also has the advantage of being an external device, so it will stay cooler (not a big problem nowadays, but this is an advantage nevertheless). But it will cost you about $50-70 more.
An internal burner will probably be more stable than USB 2.0 (kinks are still being worked out on USB 2.0, but it's just a matter of time
) and it will cost you less. BTW, this Plex 40x drive is higher rated (besides being faster) burner as well.
So you will have to ask yourself if you need portability or no...as I see this being the main advantage of the external Plex.