I had the same dilemma!
So I now have both! The xdj-rx is an awesome little machine and does everything you would want for mixing two decks, with the quality effects and features of most top end pioneer cdjs and mixers. Also not too large in a flight case for moving around and taking to parties/gigs.
The Ddj-rz on the other hand is equally awesome and more! I end up using this over the rx most of the time as I can progress to using the 3rd and 4th deck and try push my mixing to the next level and bring in different elements instead of just going from left to right and back again.
Which ever you choose, you'll get a lot of fun out of them!