Ive never used the nikon lens, but ive had canons 100-400 for 3 or so years now and taken many pictures with it. Its not nearly as sharp as the super zooms, but for the price i dont think its reasonable to expect this. Its a great size for carrying around though and im very happy with the iq of the images. i use it alot for birding, but its not really very good in this regard, action like flight can be hard to capture but under the right lighting condition stopped down to f8ish, i like its sharpness and i have some images i really love from this lens. Since everything is a compromise, this lens suites my shooting style and budget well.
Many people aren't a fan of the push pull design to the zoom but i like it alot. When searching for a bird in thicket or such, it allows quick zooming out to see it and then its easy to zoom back in after finding it.
I wish they would bring out a newer version with weather sealing and a new generation is system though.
To the point of your original question. I think the push pull zoom and the usm focusing system would be worthwhile over the nikon. But ive never used this lens so its just some surmising on my part.
I guess my advice, if you are trying to decide between the two systems would be to wait until the new nikons are in stores and compare the cameras of your choice before deciding. I personally shoot canon because i have grown to like and enjoy the iq of their sensors and the handling of the bodies. The competition is very close now though and likely at this point down to personal ergonomic preferences. For what its worth, ive shot both in the past, i shoot exclusively canon now. Good luck!
