I prefer Nikon simply because I like the way their cameras feel in my hands, handle and the user interface. Having said that, I now work with both systems - Nikon has nothing in the 400 AF F4 or F5.6 arena. Yes the 200-400 zoom (too heavy to hand-hold and lousy with a TC); and the old 80-400 - neither cut it image wise compared to Canon primes (400mm). Nikon has to do something to address this otherwise I may just jump ship altogether - after all I can use the good Nikon glass with adapters on Canon bodies. In the meantime I will shoot the Canon af F5.6 and the DO F4 lens for birds in flight. Wonderful lenses both - Nikon needs to address the wildlife shooters who need high image quality without the weight (read F2.8 max aperture). Many people need ease of movement with telephotos for birds in flight not haevy weight telephotos...and with the ability to shoot at 800-1250 with low noise, the faster F stops are not always needed - unless you want to completely blur the background.
