Hi all.What are your thought on these two at 600mm?The nikon becomes a 600 5.6 at 600 with the Sport being 6.3.Of course there'll be no comparison at 300 but how about 600?Ive never used TCs so if you have any info it would be great.The Nikkor obviously being the much more expensive but (possibly) more versatile.Whichever I get it'll be used on a d810