Maxtor sells the same drive under their own brand name with a list price of $399. I have read great reviews of it and know some Systems Engineers at Apple who recommend that drive to customers wanting to do iMovie/FCP. If an Apple SE says it is good, I tend to trust them.
I don't know what enclosure TransIntl.com uses for their drive. That is the only difference I can think of. The enclosure does make a difference sometimes. I would buy the Maxtor version because it is the one with the great reviews. The catalog places have it for a little less than list so you'll get the same deal on it.
VST's larger capacity drives are more expensive because they use IBM's Deskstar 7200rpm drive mechanisms. Those are significantly more expensive than the Maxtor. You get what you pay for. The IBM's are really fast.
However, the 5400rpm drives can keep up with digital video so your real world results wouldn't be all that different, especially when you consider that FireWire drives use a basic EIDE to FireWire bridge that limits the performance of your drive to a max of about 12MB/s, well less than the performance a 5400 or 7200rpm drive can sustain when plugged into an ATA66 or ATA100 controller.