It depends, a little research is normally needed to find the model of drive inside the caddy.I still have no clue why external HDD's are cheaper. Do they run at 7200RPM and have the same cache?
I know some WD ones have the slower 'green' drives fitted.
In my case the drive was the Seagate Barracuda 2TB 7200RPM SATA 6Gb/s 64MB Cache ST2000DM001, the same drive sold as bare OEM.