The 5DIII is better than the 5DII in every way, soup to nuts. Not always hugely better, sometimes even only insignificantly better for certain applications, but always better.
...except, of course, for price.
So, if price isn't a factor -- and you seem to be indicating that it's not -- then it's a no-brainer.
That writ, studio work, where, presumably, you're shooting with well-balanced strobes at ISO 100 and f/8 - f/11, the only time you'll see a difference with any camera on the market today is when printing on a machine that's too big to sit on a desktop (or when pixel peeping). Resolution isn't a factor at desktop printer sizes, and dynamic range is a moot point when you can push a lever or position a reflector or whatever to tame the light to whatever it needs to be. Noise, of course, is also a non-factor at ISO 100. Keep going down the list and, again, unless you're printing big, only ergonomics really differentiates a Rebel with a kit lens from a 1Dx with L glass in the studio.
(Again, just to be clear: I'm assuming studio strobes in typical use and 13" x 19" prints and smaller, no significant cropping, etc., etc., etc. Atypical use of strobes, window light, all sorts of other factors could easily skew the equation all sorts of ways.)