Just made my mind about going with a D600 coming from a D90. My dillema now is what to use with the new body. Initially I was decided to get a 24-120 f4, but some mixed reviews about the performance at the long end made me think twice. For a similar price I could get the kit 24-85 VR and get a 105 2.8 from Sigma (or either the Nikon with some more $$). My key concern is performance for portraits. On the D90 I was happy using the Tamron 28-75 2.8, around 70mm, which is equivalent to ~105 on the D600. Would the performance of the Sigma on the long end be enough better than that of the 24-120 for me to give up the convinience of that as a one lens solution? I am not a professional, the level of performance I need is to get great pictures of my kids.