I shoot with both and the 1DX whips the crap out of the 1D4, IN LOW LIGHT. At low ISO's, not really much difference in real life. Since I have sufficient glass, the extra reach isn't a huge concern but it helps. I put either a 300 or 400 on the 1DX and the 70-200 on the 1D4 and is fantastic combo. The 1D4 cannot be useable above ISO 5000 in my opinion whereas I had keepers at a soccer match of ISO 16000. I didn't really notice any real difference in AF accuracy to tell you the truth. I had just as many misses with both cameras in the low light. However, the IQ is different beginning about ISO 1600 and above. Overall, the 1DX is better, with the 1D4 coming in a close second. The differences exaggerated on spec sheets aren't realized in real life in my opinion and that's exactly why I keep both cameras with me at sporting events. Just evaluate where you'll be shooting. If there isn't much light, like indoor sports or night field sports, the 1DX is superior.
I took the plunge and sold my Mk IV for the 1DX. I shoot some sports but more fashion runway type stuff. I agree that the ISO performance of the 1DX is superior but I have yet to get the most out of the new AF system. I'm still experimenting with tweaking it. Especially AI Servo doesn't seem to lock on reliably. I'm mainly shooting One Shot 90% of the time because I can't trust AI Servo for paid jobs yet. Would you mind sharing what settings (Case) you use and/or if you even use AI Servo? Thanks.
Sure. I shoot mainly sports. I use AI Servo for all of my stuff. Typically, I use Group A lenses, so that I can access all the dual cross type AF sensors. Now, during sports I'll use any of the center vertical points trying to stay with one of the 5 dual cross types, with 4-pt expansion and using either Cases 2, 5, or 6. I will move my "box" around the viewfinder, but using what I stated, I haven't missed focus yet at all in a sporting event. One soccer game I shot 356 photos, all in focus. I leave all tracking sensitivities set in the middle as well.
Fashion shots? Take advantage of the face recognition feature if you want to focus on the faces of your subjects (I don't know where you focus or how you meter, so I cannot tell you for sure), but if I did that, you can then use ALL 61 points in AI Servo mode, because you can select which individual point to START with, then lock onto the face and the AF system will never let go of the face, and the active AF point will simply move around the grid in the viewfinder staying locked on the face. I'd definitely practice these features before a paid job obviously though.
These are my basic settings that I always use.