I am considering an upgrade to my equipment
1D M4 -> 1Dx
500F4 IS -> mark 2 version of this lens
Already have the 5DM3 but love the feel of the 1D series, though it is a lot of $ for a body.
Mostly shoot wildlife with the 1D and use the 5D for macro and scenics (the tiny bit I do of city scapes).
Really don't need any new lenses - in fact, I need to have a garage sale and join a 12 step program for lenses and bags.
Originally I was very skepical about the 1Dx but the more I hear about it, the more I am interested. I will be borrowing one for a shoot next week, so I will have hands on experience then.
I could also wait, to see what Canon introduces this fall.
High MP camera (probably a replacement for the 5DM3, not sure if it would be wildlife camera)
200-400 zoom in lieu of long lens
Hi RGF, My thoughts based on I shoot Wildlife, I own the 1Dx, 5DMK3, 1DMK IV.
Get the 1Dx, just the viewfinder alone is worth it, especially in Low light, You have the 5DMK3 so you know how much better the 61 Point Focus system is compared to the 1DMK IV, then with the 1Dx you have the added ability to enable colour assist on Focus, so I find when I'm shooting say a Cheetah at 80kph against a blurred brown background, the contrasting colours of the Cheetah make the focussing more efficient.
You give up the 1.3 Crop of the 1DMK IV, which does give your 500 Lens more reach (650), but you would be better to sell your 1DMK IV and use the new Version 3 1.4x Converter, I don't use the converters much at all, but the 1.4x when I do use it seems to have little to no impact on the IQ.
Wait for the 200-400f/4 (1.4x), your 500 is a wonderful lens from all reports, but the 200-400f/4 (1.4x) should be a Giant Killer, I've played with the Lens in Singapore in January this year, it's a bit of a Beast, but I own the 400f/2.8 V2 and I don't think there's any difference between Size/weight & IQ between the two, IQ obviously is a "feel" as we weren't allowed to use Cards in the Camera, but it's worth waiting on the lens change out to see how the 200-400 performs, I'll certainly get one as I have one Pre Ordered, so happy to keep you informed. In the meantime your 1Dx + your current 500 is still an amazing combo.
I think the High MP Camera will be late in 2014, I also feel/hope it's a 1 series Body as I tried the D800 and among other things the 4 FPS was a killer for me, so hoping the new High MP Canon will be 1 Series Body (For weather Proofing), 40MP, 7 FPS, then you have a great Combo. 1Dx for 12FPS, and the above High Mp Body for when you need IQ over speed.
Get the 1Dx you will be absolutely happy with the decision.