Aaron,
Though not an Elinchrom user, aside from their Rotalux Octaboxes, the first obvious difference between the Ranger RX and the Digital RX systems is power source. The Ranger system is battery operated for portable use. The Digital RX is an AC/mains power pack to which you connect heads. The output, recycle times, total power output, range of power control (number of stops), etc., will be greater on the pack and head system than the battery powered system for the most part.
Elinchrom has an updated site (
http://www.elinchrom.com/) that has a lot of very good information on their systems. It is definitely worth the look if you are interested in these very good lighting systems. Others, like Guy or Bob, may be able to add more, since they use Elinchrom stuff. I have Profotos, but do use the Elinchrom light modifiers for some of my shooting, just not the heads, packs, or battery systems. If you need portable, the Elinchrom Ranger RX or their new Quadra system is very nice. If you are in a studio, or setting where you have power, the Digital RX packs with Digital heads would be a good choice also.
LJ