I own a 5D II. I am intrigued by the III but only because it has in-Camera HDR and more than 3 stops of AEB (up to 7 stops..why couldn't Canon have given us more stops on AEB 6 years ago is what I want to know? MANY cheapo cameras have offered this for years!)for when I shoot images for HDR. I have to use a Promote camera controller now which is a major pain in my %&!.
Anyway...what I have decided to do is sell my 5DMarkII kit lens , the 24-105mm f/4L IS(a great lens but not as sharp as my primes...but no slouch for a zoom) and buy the new 24-70mm f/2.8LII, to use on my current 5DmarkII. I do not think I need to run out and get a 5D III for improved image quality. I doubt that their is any "noticeable" improvement. I think that the image quality of my 5D Mark II is just fine and will shine with this new zoom. If this new zoom is as good as my 70-200mm f/2.8L IS II....then I will be VERY pleased and I am probably going to be making better images than I would by buying a new $3500 camera body, and I will be gaining an f/stop for all subject matter, not just non-moving objects, (the IS takes care of that on the 24-105). Just my 2 cents. I am not familiar with the 5DI, but I hope you arrive at the right decision for you without the influence of a salesman...that can be suspect.