Just bought 5D mark III - heard Nikon D800E is much better image Quality? did i make a mistake?

I wouldn't think so.  Your camer is top quality and you would likely never notice a difference one way or the other. 

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One day long ago, a sorcerer and a warrior did battle, as such things were commonplace in that age. Sometimes the sorcerer won, and the sum value of human abilities was improved some trifling amount. Sometimes the warrior won, and again the human race improved by some insignificant amount, for a sorcerer who can't defeat one miserable warrior is a poor sorcerer indeed.