Both chips are 13.56MHz and ISO14443A compliant, but they have dramatically different memory structures and features. The xNT is based on the NTAG216 chip which is also NFC Type 2 compliant, and the xM1+ is based on a Chinese clone of a Mifare Classic 1k chip, which is not NFC compliant.
Many legacy systems still use the Mifare Classic, so the xM1+ might be useful for people who want to clone their Mifare Classic 1K cards to an implant. If you do not have a specific need for the xM1+ I would not bother.