Since I got my 1d4 I've done almost exclusively wildlife and sport. I love the mk 4, fast responsive, an excellent all round choice. A few weeks ago I needed iso performance of 6400 and the weather was awfull and I'm afraid the grain and pics just don't cut it. I was wondering, if you had a mk4 and bought a 5d3, did you miss the speed and crop factor? If you bought a 1dx did you miss the crop? I won't change my 500 lens as I shoot bigger animals and will still have the mk 4. Oh and before I forget, in real world shooting, what's the picture quality like 5d3 or 1dx compared to your mk4, not iso charts, real world shooting. Tell me not to bother, don't upgrade and my wife will love you forever in keeping my bank balance in check.