I'm in the exact same situation, but i bit the bullet at the weekend and ordered a Sigma 150-600 C, i figured that the Nikon is the best part of twice the price of the Sigma and from looking at the images certainly not twice the lens in terms of IQ, plus the Sigma has more reach, and the added bonus of the Sigma USB dock to tweak the lens to suit
Should turn up this week as ordered from DigitalRev............. hope i haven't made the wrong decision
Also, in buying the cheaper Sigma it allows me to keep onto my 300mm F4, had i gone the 200-500mm route i would have had to sell/part-ex my 300mm in against it