i would absolutely go for the 70-300mm sigma if they put OS in it... i had the 55-200mm vr nikkor before, it was sharp.
this picture was taken using nikkor 55-200mm vr. sadly i have to sell it away to fund an upgrade, because of some fraud story regarding 2nd hand equipment which i will post on another thread, anyways... am just wandering wether the sigma 50-200mm hsm os would be faster in lowlight, coz the nikkor 55-200mm vr is slow in lowlight condition...