I haven't compared the two but I have home trialled the CXN and investigated the CXU. What I would say (and this will sound pretty obvious) is that if you want a disc player, the CXU is the right machine whereas if you want a streamer, the CXN is much better.
The reason for the first statement is pretty obvious, in the case of the second, the CXU doesn't support app control for music streaming. I'm amazed because I assumed it would be the same stream magic module as in all the other streamers, but apparently not. If you want to use it as a streamer, it needs to be hooked up to a TV (not a problem if you're using it for DVD/BD anyway) and then control is via the remote. Whereas the CXN can be controlled using the Connect App (which I find very stable).