You can get two of those 1.5TB USB drives for the price of one 2TB FW800 drive. As stridemat said above, the question is, do you need the FireWire? If it's used for backups, you should be fine with USB
If I remember correctly, the My Book Studio II is in a RAID setup, so that makes it more expensive. You could just get a regular My Book Studio 2TB, as that's only one drive, and goes for 130 euro. I have the 1TB version and it's awesome so far.