I think Canon or Nikon will be better at focus tracking, and some reviews also point in that direction. Part of this is because of the lenses; SDM doesn't help here. I have the impression that DC is already a bit better. Not so much because of the pure speed, but because it requires fewer micro-adjustments. But that's just my impression from playing with it in a store for 5 minutes.
In any case, I prefer the Nikon user interface of the tracking AF. Nikon always highlights the active AF point in the viewfinder. Pentax only highlights it when it focuses. So with Nikon, it's easier to determine if the camera is still tracking correctly.
Also, Canon/Nikon's more expensive lenses generally focus faster. And I have trouble tracking my 4-year-old niece with my K-3.
A few weeks ago, I contemplated getting a used D300S or something, just to be able to draw my own conclusions. The event that triggered that was a motor race I tried to capture with my K-3, 60-250 and 1.4x TC. But in all honesty, this was the first time I did such an event and I shot through a fence. It got me the composition that I wanted, but it may have affected the AF.
---------- Post added 07-21-2015 at 01:42 AM ----------
Originally posted by hcc Do not believe all threads. The K-3 AF is more than decent: "any enthusiast photographer that picks up the K-3 (with a silent-focusing lens) will not be disappointed"
Pentax K-3 Review - Autofocus | PentaxForums.com Reviews
Not that I don't value the reviews which are posted here, but Pentax fans reviewing Pentax gear will probably want to say positive things about it. There are other reviews out there, such as
Pentax K-3 Review: Digital Photography Review.