The two implants contain different chips. The xNT contains the NTAG216 and the xEM contains a ATA5577 RFID chip.
The xNT uses the 13.56MHz frequency band, and is fully NFC type 2 compliant, which basically means you can read/write to it with a NFC capable smartphone (Most Android smartphones), and make smartphones do certain things when your implant is scanned (such as showing your contact info, opening a specific app, etc.). You can also buy door locks compatible with the xNT(ISO13334A standard) and unlock your door.
The xEM uses the 125kHz frequency band, and you can clone certain access cards onto it, altough that requires a seperate cloner. The xEM can’t be used with a smartphone.
Also, Amail has a FAQ up which explains the difference between the different chips in greater detail, as well as a bunch of other questions your might have.
Stuff like writing your contact info to the chip is pretty easy to do and installing a compatible door lock is just a matter of changing the lock to your door.