The NExT is the best option for general applications. The xDF2 is a DESFire EV2, which is backward compatible with the EV1 and original DESFire chip… but there are caveats.
The biggest issue advantage of the EV2 is that you can have applications set up on the chip that do not require giving up the master keys to set those applications up. The down side is that almost no systems are designed to share the chip with other applications not managed in some way by the system you want to use it with. For example, there is no way most transit systems that use DESFire to allow their payment application be set up alongside other applications already on the chip. The world is just not set up to take full advantage of the technical capabilities these chips have… which is why the Fidesmo approach was so interesting to us and why we partnered with them for the VivoKey Flex One.