Do you mean Nikons 24-70mm F2.8...If yes this Nikon is F2.8 through the length, where as the Sigma is F2.8 at the short end a F4 at the long end...I think it would be a reasonable lens...
If you are thinking of getting it, is it compatible with your camera...Some lens wont auto focus on Nikons D3000/D5000 series cameras