Tamron 150-600 is very good... also are you aware of the aperture weirdness on the Sigma, esp if you were considering the sport version.
http://alanhewittphotography.co.uk/...budget-tele-zooms-sigma-and-tamron-150-600mm/
The sports version is much shaper than the Tamron and moreover the focusing on the sigma Sports is well ahead of the pack.real world usage will.mean 90% of photos at 600mm f/6.3 and most of the rest at 150mm or around 400-500mm, i.e. you nearly always will be at max zoom or somewhere where the apertures are the same. It is disappointing the much heavier more expensive Sigma S is the slowest but optically it is the best.
Buy for Nikon one has to seriously consider the 200-500mm f/5.6 because it is in a newly sharp for the price.