For some reason, more people than actually own a high-end sports camera are arguing about the answer to this question today. It's actually a question that has an easy answer:
- If you were shooting with a Canon 1DX Mark II — get the 1DX Mark III or keep using what you have.
- If you were shooting with a Nikon D3 through D5 — get the D6 or keep using what you have.
- If you're new and don't yet have a sports camera — seriously consider the Sony A9/A9m2, though be aware that the 20 fps mode has lighting limitations you need to consider; it's really a 10 fps camera for a lot of sports. But the Sony is still the low cost entry point you'd want to start your buying decision thinking about.
Any other answer is wrong, in my opinion. That's because both Canon and Nikon have demonstrated that their deep set of EF and F lenses will slide right over to a mirrorless system user when they finally make a mirrorless sports camera. Both are all remaking their key lenses in the mirrorless mount, only with better optics. Since one good sports lens costs more than the body does in the first place, you don't want to trigger a costly cascade of buying by switching systems. There's not enough dollars left in the sports photography client base to justify taking US$20,000+ hits to switch systems. That might have been true in 2005, but not today.
Photojournalism may be another story, but frankly Canon and Nikon have low-cost mirrorless answers that let you slide out of the top-end DSLR to mirrorless, too, and 11 fps and 24mp is more than enough for most of those folk (e.g. Nikon Z6).
Likewise, APS-C may be another story, as well. Canon never updated the 7D Mark II, so it has a bit of a disappointing sensor in a still-nice body at this point. Nikon never updated the D500, but frankly, it's still a pretty sweet camera at the top of the APS-C chain. I'd take it over the Fujifilm X-Anything, and mostly due to focus performance and lenses suitable to sports tasks. I'd take the D500 over the Sony A6-and-three numbers models, as those models just don't have a natural UI that won't get in your way while shooting sports, and again, the lens choice for the D500 has options you don't get in mirrorless (e.g. the 300mm f/4 PF).
Meanwhile, I'm hearing from lots of D5 users. The wildlife shooters are disappointed. I understand this, as without actually trying the new focus system, it doesn't look there's anything else in the new body that would be of interest to them.
The sports shooters pretty much went from "I'll almost certainly update" to "I'm taking a wait-and-see attitude." I blame that one on Nikon marketing. The actual marketing line probably should be "Same state-of-the-art image quality and body construction you're used to, but now with even better focusing ability and control, plus we've added lots of your handling and workflow feature requests to make your job easier."
Unfortunately, that approach really says that the D6 is really a D5s, something Nikon is too proud to admit.
One other thing that isn't getting noticed by most is that Nikon has been executing on eight-year cycles for decades now, dating probably all the way back to the original F. On one visit to Tokyo HQ in the 90's, I was shown a chart that seemed to verify that (both historically and projecting into the future).
What that means is that the D1, D3, and D5 were the "big" change cameras on eight year cycles. The D2 and D4 (and now D6) were the "smaller" change cameras on intermediary four year cycles. The "s" models were "smallest" change cameras on two year cycles. Put another way, it's the D7 (or perhaps Z9) that we're all waiting for the next major change from Nikon with.
I actually like that approach. What it means is that you take your very best engineering teams and have them make a statement, then they go back into the labs and do deeper research and experimentation to try to figure out the next big breakthrough while your other engineering teams iterate on the base they were given.
