1d mark iv instead of a 5d mark iii

Yes, the 5D3 certainty does improve on the 1DIV AF keeper rate: http://m.dpreview.com/news/2012/08/01/Roger-Cicala-Lensrentals-Canon-Autofocus-marketing-claims

It's not just about the keeper rate; it's an all new AF system in the 5D3. The 1DIV was just another chopped up version of the old 1D AF, and canon put so much emphasis on rectifying the 1D3 Tracking debacle that they actually flubbed it in other areas - percision and low light bein the worst. By percision, I don't mean accuracy, but rather the monolithic AF points that woul catch anything even slightly contrasts anywhere in the same hemisphere as your selected AF point. I couldn't tell you how many times I would think the AF had an easy target only to find that the damn thing would hit whatever object miles behind the subject. I got to the point where I simply and literally could not perform group shots with it, having to resort to the 5D2 instead.