Hi Olliholms,
Remember that the Sigma f/2.8 should work with the 2X TC and maintain AF but the Nikon f/4 will not. Personly I would be tempted to go for the Sigma; I am a glutten for speed. Try to get some other feed back on the Sigma, if it is a prime lens and not a Zoom, I think it will be your best bet.
Best regards
Jonathan
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