Hey, I know how you feel. Just take a deep breath. I have owned all 3, the 5D Mark III, 5D Mark II, and 7D. I learned to use all 3 and loved all 3 once I did this. I did sell my 5D Mark II and 7D to get a new 1D Mark IV. The reason I mention that is, I did not get rid of those two because of anything negative. I'm not the guy to ask about post-processing software, as you may have seen some of my posts regarding that, but, I think I can offer a solid opinion here. If I were you, personally I'd get the 5D Mark II as back up. But, what is it that you shoot? Do you need fast action, fast autofocus? Well, in general, the 5D Mark II's AF system is not nearly as bad as you might think or have read. Remember, the only people posting about the AF performance are the ones who can't figure it out. Just like I was the only one who complained about LR 4, because I was the one who couldn't figure it out. Put it in AI Servo mode and use the center AF point and you're GREAT. I shot a lot of railfan photography with the 5D Mark II and as long as my shutter speed was 1/500, they were in focus and tack sharp. In other words, nonesense. Once you get the AF figured out, IQ is just as good as the Mark III and you won't regret that decision. It is a gem of a camera and I'm personally going to get one as a 3rd camera, as a backup. I really miss that camera.
Other scenerios, if you really don't trust the AF and just think you can't live with that, I once considered buying a 1D Mark III used as a backup sports camera, and could have bought one in great condition for $1700. It's only 10mp though, so think about "blowing up" photos. Also, the burst rate on the 5D Mark II is only 3.9, vs. much higher for 1D Mark III. My advice is to keep your 5D Mark III and buy a 5D Mark II new from B&H as backup, test it out 30 days, and return if not satisfied. A 5D Mark III/5D Mark II combo will cover everything and more.
