If DR is your primary concern, go Nikon. If you are going to stay with Canon and get a 5DIII, I doubt you'll find a more versatile body in existance. I can't really say my composition, creativity, etc are better with the MKIII (just like they wouldn't be better if I got a D800), but I can say that there are a whole bunch of shots I couldn't get on my older bodies that I can get on the new one because of its low light ability, and that between the focus system and higher frame rate than older FF bodies, this one camera easily does the job of my two older ones (5d and 40D).
Spec sheets are deceiving. Its easy to make the grass look greener on paper. My guess is a lot of people who switch systems either way will find, after all their horse trading, that the end result isn't that much different. There are a few people who do so with good reason and really take advantage of the true differences in the system (like maybe tilt-shift availability, supertelephoto availablity for canon, or the few people who really NEED the extra DR of Nikon). And there are a few people who will do so and pull up every shadow 4 stops just because they can, and be happy. The rest...will be just where they were when they started.