Lens question: Nikon / Tamron 24-70 or 28-75 f2.8, which ?

Ok, now that I got my D600, cant wait to get a FX lens for midrange.  I really cant decide between Nikon 24-70 f2.8 or Tamron 24-70 or even 28-75.  Does anyone have any feedback as to which is the best option considering price / performance.  I know Nikon is much more expensive.  Is it really worth more than 3 times of Tamron 28-75  which seems to be a better value since it's much cheaper than the other options.  Anyway if anyone can give me any guidance to this new FX newbie I'd appreciate it.

--
Alex