I upgraded my 5D3 and 1DX to the 5D4 and 1DXII, mostly because of my gear acquisition affliction . For action shots, the focusing is definitely improved and I almost never miss a shot. The 5D4 with the new 100-400 zoom is a ridiculously sharp combination --- I've been thrilled with the results. I used both cameras for candid shots at a wedding recently and they did very well, although the 1DXII was much too loud in the ceremony for my liking. I probably should have switched to silent mode, although it only helps a little. The 5D4 is definitely quieter. I love the CFast cards in the 1DXII --- they are crazy fast and the buffer never ever fills up. For wildlife shooting it's a dream come true. But yes you'll need to buy lots of cards (and bigger hard drives too) to store all the extra photos you'll be taking when you shoot bursts of 100 shots in a row!