I have the AFS II 600 and my fiend the VR - I've been meaning to finish a comparison article on my blog for ages but just not got round to it. But in the meantime, I can tell you this:
The VR is much better balanced so feels lighter and easier to carry. The VR is also a tad longer.
The AFS II can have the focus ring switched off for beanbag shooting, which is brilliant, why they didn't keep that feature for the VR I'll never know.
Both lenses focus just as fast with a 1.4 on as they do without, so you get a VERY useable 850mm f5.6
In terms of image quality...the VR has the slight edge, but in real world shooting there is very little in it. When I've gone out with my friend, both shooting D3's and come back at the end of the day our images look pretty much the same in terms of contrast/saturation etc. The VR is a tad sharper but again, not a massive amount in it.
For me it came down to this...is VR worth £3k (as that's how much more I would have had to put in on top of the
second hand AFS II I got from my friend when he upgraded)? The answer was NO. Yes VR on a 600 is very useful, I won't argue that - I've managed a sharp shot handheld with a 1.7x on - which is bloody impressive. If I had unlimited funds would I have gone for the VR? Probably.
But the reality is the older lens is every bit as capable if your long lens technique is up to scratch. And that focus ring feature is a godsend if you shoot from a beanbag.
Personally, a 500 would be easier to travel with but with wildlife you always need more reach so I figured if I was serious about it I'd put up with carrying the 600 around - at first it seemed like serious effort but now I don't give a second thought to carrying it around as I'm used to it. And as you have the 200-400 like me, it doesn't seem worth buying another lens that only gets you 100mm closer.