The mifare chip in the xM1 is not NFC compliant. Some phones will be able to read it, but only certain android phones with NFC reader chips from NXP. Also it has a 4 byte ID. The xM1 mifare chip also has a programmable ID.
The ntag216 NFC chip inside the NExT and xNT is a fully NFC type 2 compliant RFID chip with 7 byte ID, which is read only.
Compatibility is what it all comes down to. They are both 13.56MHz and both ISO14443A compliant, but so many systems that will work with the xM1 becatits mifare won’t work with the ntag216 in the NExT, and vice versa… and if you want reliable NFC operation with phones, the ntag216 is a must.
As for the reprogrammability of the xEM, the product page talks all about it.