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The 600EX-RT has an AF assist pattern that matches the 1DX and 5DIII AF sensor matrix. I found it's better (i.e. faster) in low light on my 1DX than a 580EX or 580EX II. So, if you're likely to shoot low light events with your 5DIII (or you get a 1DX), then it might be worthwhile. Otherwise, maybe not. Also, the 600EX-RT has a longer max. focal length of 200mm. This might be a plus for long lens shooting without a Better Beamer, but I haven't tried it yet.