So far I only have one legit shoot under my belt with the 1DX hanging from one shoulder and the 5D3 from the other. I shot a choir concert in a poorly lit church, so I got to work out the high ISO. I just started editing the photos but have a few observations so far.
- Image quality seems to be equal or so close that it doesn't matter to me.
- Now that I can limit the range on both cameras, I shot on auto ISO (only done that a couple of times). I limited the 1DX to 25,600 and the 5D3 to 12,800 based on some other reviews I have read. All of the shots seem to be very usable. When I am done editing, I'll have a little better idea, but I never went beyond 4,000 on my 5D2, so this is interesting new territory.
- AF performance seemed to be equal. I occasionally switched to AI Servo because there were some dancers and had a few misses on the 1DX -- I rarely use AI-Servo so it might be able to my technique to fault.
- Shutter speed/feel/response...I cranked up the low-speed continuous shoot to 6/sec on the 1DX (it's programmable) and did a lot of multiple-shot sequences and really found it to be responsive. On the 5D3 I only use the silent mode, and that's about 3/sec so it's automatically slower and it's sort of apples and oranges. I suspect the full-speed 5D3 isn't so different that the low speed 1DX. I will say that the lack of a real or even usable silent option on the 1DX is annoying the heck out of me. For weddings or quite music concerts or anything quite for that matter the silent mode is a godsend.
- Exposure...this one is the biggest difference from my viewpoint. The 1DX nailed the exposure in almost every shot. I'm very curious to see how it handles flash shooting, but the 5D3 was way off on a lot of exposures while I was shooting. If you don't shoot very often in auto modes, this won't be a factor though. Even with the noisy shutter, I ended up using my 1DX almost 90% of the time on the shoot because of the exposure accuracy. It was very mixed lighting, so having it accurate without compensation was very nice.
If you already have 2 5D3's I would advise you to stick with what you have and you shouldn't think twice about it. The value of having 2 of the same cameras to shoot with is huge, so upgrading both would be an extra $6K investment for not a whole lot extra. If you were shooting sports it's a no-brainer to make the leap, but the 5D3 is a superb wedding camera and that silent mode is major bonus.
I have a tennis tournament to shoot in Sept, so maybe that will settle me on the 1DX. By then, I'll probably upgrade my 70-200 to the new version as well, so it should be a dream combo. At 12 frames/sec I should be able to catch some stunning moments of play.