Actually the Nikon D3 has surpassed the Canon cameras at this time (since the China Olympics actually). It is the extreme low noise at high ISO that makes it so popular with the sports photographers. You can by a new D3 for what a MkIII costs used ... and Nikon makes an excellent 200-400 mm f/4 (with the low noise, you no longer need a $7,000 f/2.8 lens).
Before you spend a dime, look closely at the two camera systems and see which will suit your needs over the next decade or so. In the mean time, buy the least expensive camera body you can find that matches the system you decide to "marry" and learn on it. In the three to five years it takes to become fully competent with that camera, the newest and much better technology will be available and you will have been able to spend the $5,000 or so saved by buying an entry level camera on fine lenses ...