The hardware between all 3 is basically the same.
The firmware for internal drives will limit the PATA communication speed to UDMA mode 2. The firmware for drives shipped as an EW162U bumps the PATA communication speed to UDMA mode 3 (oddly, not mode 4*) to ameliorate a bottleneck that results from inefficiencies in the USB-to-PATA bridge chip and allow burning at higher speeds in the right conditions.
If you use the drive internally, you can use the internal firmware and be fine. If you use the drive externally over something like USB, try out the internal firmware and see if there are any bottlenecks.
I also believe the DW1620 firmware was kept more up-to-date than the EW162U and DW1620 Pro firmwares, though there are various quirks that you will have to test for yourself.
*: Irrelevant history lesson: when BenQ released new hardware as the DW1640/EW164U, the EW164U got UDMA mode 4 (maybe the hardware finally supported it?), which was standard for pretty much all other drives of the time, but the DW1640 internal firmware was still limited to UDMA mode 2.