Your choice is between the lighter, less expensive, and easier to travel with 500II and the heavier, bulkier, more powerful 600II. I will be doing a blog post on that in about two weeks or so. I hope.

Hi Melissa, Both the 5D III and the 1D X blow the 1D IV out of the water when it comes to high ISO performance. When I worked the images for this blog post, increased background noise was shockingly evident. I did do those conversions in ACR. I need to try one in DPP following the stuff in my DPP RAW Conversion Guide. For two examples of the 1D X at ISO 1600, see the 600II vs 800 blog post here, especially Arash’s White-tailed Kite image down near the end. There is a link to a large version; it is jaw-dropping.