I would suggest you arrange a demo of both, make up your mind and just go for whichever one sounds better to you. I haven't heard either, but I am pretty sure they will both be very good.
Then, and this is the important bit - unless you encounter any problems with your set-up that require some help from fellow forum members, I would suggest that you stay away from the 'Streaming' section of this forum for a bit (at least from any posts that make reference to a comparison between the ND5 XS2 and NDX). There will always be someone who posts to say that the NDX completely 'blows away' the ND5 XS2 or vice versa, when in reality the differences are likely to be much more subtle, and this just may begin to sow doubt in your mind about your own choice. Just get your device up and running, and then spend a couple of months or so getting used to and just enjoying your streamer before getting fully back into the streaming forum fold.
Whichever device you purchase will be very good indeed, and I am sure you will be very happy with it.