I didn't realize the Nikon was so (relatively) new. Still, it's the substantially lighter weight that, IMO, sets the Canon 600 II apart - it's the same weight as Nikon's 500/4 (and the 'old' Canon 500 MkI)
But alas the current 600mm is perpetually out of stock in most places so you may want to consider looking at other focal lengths to get to where you want.
Actually, I need look no further than my closet to find a 600 II. 
So YOU are why the new lenses are never in stock!!!
