Paul - The 300 2.8 Nikkor is nice, Im not saying it isnt.. I've used both side by side for birding and motorsport, I found the canon much more reliable for focus accuracy, and a hairs width faster to focus.. There isnt much in it...
The only, and I mean ONLY reason I don't use a Nikon camera much these days is the glass... the 70-200 F/2.8 VR IF ED is better than the canon 70-200 2.8L IS, faster to focus, better colour reproduction lighter, (although somewhat lengthier) the wider Nikkors and wider Canon's I have little to no experience with.. I do the majority of my terrestrial photography at over 200mm F/L... never below 85mm..
I've used the 300 and 400 2.8 canon's with the 1.4x TE, they both functioned as if the TE wasn't there.. obviously requiring lower shutter speeds, but accuracy and speed were relatively unchanged... IS took up the slack when my arms weren't stable enough to hold the lenses still!
