I'm not a Canon shooter, but I have an opinion anyway. (Surprise, surprise!). It seems to me that if you can afford it, go with the Mk III. From all I can tell it's a good step up from the MkII. On the other hand, I strongly suspect that in normal day-to-day shooting, you probably wouldn't notice a whole lot of difference in the results, UNLESS you shot most of your stuff at very high ISOs. If that is the case, then there's no question that the MkIII performs better - indeed, essentially better than any other camera of this type. I can understand why you're wondering, because I think most of us have faced similar delimmas in our purchases over the years. From my experience, I can tell you that I've seldom, if ever, regretted buying gear that cost more, because it almost always offered advantages over its cheaper brethren. So, I don't think your question truly has a definitive answer; I think you have to decide for yourself whether you would be truly happy with a MkII while knowing that the MkIII is also available. Good luck.
Have you considered the 6D? If you're not going to be shooting video, that might be the call for you. You get a lot of the image quality (in some cases better) than the Mark III with a price that's only a few hundred dollars more than the 5D II. I went with the Mark III because the AF totally spoiled me when I shot a wedding with it and the 6D didn't cut it for me personally. But, it's about the same as the AF on the Mark II, so it's kind of a wash there.