According to JPI there's a circuit board addition in USB version. I presume it includes a USB chip but I suppose it could be be an internal serial-to-USBadapter. Either way having the USB version means not having to drag a laptop to the aircraft to download the data and possibly a USB-to-serial adapter. A USB memory stick is much more convenient. Even in flight I can download the data conveniently.
The other option - an adapter they sell - is a serial to USB adapter box that needs to be powered from a cigarette lighter while in use. I think it's a clunky solution compared to sending the EDM away to be modified with a replacement USB interface.