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

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If you have all those good lenses, you would probably take a large hit selling those lenses and replacing them with Nikon equivalents

If you are used to Canon cameras, you might take a little while while to get used to how the Nikon camera works. Which camera feels and operates better to you? That can be important.

The 5DMKIII is said to have good autofocus, good high iso, and has a good reputation for video.
Considering that you already own a 5DMkIII and all those lenses, I don't think there is any compelling reason for you to switch