When you compare just the camera body, I do not think you see much different. What I mean is using Sony Camera, with the appropriate lens, you can capture good action photos. Same thing with XT or XTi or even with Pentax DSLR camera.
For me, the A300k is not cheap (yes . I am cheap

) Just get the camera that feel the best in your hand and within your budget.
For what you like to do, I think all the camera you mentioned should work. However, I found that ... if you go with Canon or Nikon, there are more lens choices out there. Third party lens maker will sure make lens for Canon and Nikon, but may not, or not yet make the same lens for Pentax, Sony camera.
But one thing though, Sony camera usually has the in-camera image stabilization. And it helps.