Nikon is very, very good for action shots. I say Canon is better for scenery and things that don't move, basically.
I'd say they're about equal. It's really whose hands the camera is in that matters. Though, I will admit that I feel my D5100 does better than my friend's XS at motocross photography.
The difference between motors and squirrels, is that in one second a bike isn't up a tree. Squirrels are all euhgiurwghirwuhgrwiuh. I guess it is all the person behind the camera too, I'm an amateur, and so I don't expect my photography to be all like "amigash ur so good heartheartheartheart". Motorcross photography is pretty cool though, do you have any cool shots that I could see?