Depends on how important the data is and how much data you are backing up. I would not use the Blu-ray discs (too expensive) or the USB Flash drive (not much capacity for the price). The internal drive will involve a little more work to install, although the transfer speeds higher and the cost will be lower. The external drive and the internal drive in an external case (basically the same thing, only thing that will vary is cost) is the most convenient method. It will be the easiest to restore to another pc.
As far as the rpm, the difference between 5400 and 7200 rpm probably will not affect you much since it is just for backups. The average seek time between these two speeds is only about 1ms different. So unless you are backing up massive amounts of data, I wouldn't worry too much about it.
