The former is basically a local version of the Xperia S, while the latter is a follow-up to the Xperia acro that comes with that awesome 720p Reality Display, as well as Japan-specific features such as infrared port data exchange, mobile wallet and mobile TV.
Both the Xperia NX and Xperia acro HD will be available in Japan from this spring.
Now you should forget about these two if you live outside of the Land of the rising Sun — Sony will sell them in its own backyard only. That’s not a problem though, since the other two phones (Xperia ion and Xperia S) are rock solid as well. And besides – you don’t need the extra features when you can’t use them outside of Japan, right?