I have been comparing the specification of the Canon EF 300mm f2.8 L IS II USM Lens v Nikon 300mm f2.8 G ED VR II AF-S Lens please see table below:-

The important numbers listed above for the wildlife photographer is the minimum focus distance which Canon wins 2m compared to Nikon's 2.2m, also the weight of the lens is of importance again Canon wins at 2400g apposed to Nikon's 2900g.

Nikon does win on cost by quite a long way, but if you want the best lens on paper then Canon wins on minimum focus, weight and has more lens elements.