I have been debating between the Tokina 28-70 AT-X Pro f/2.8, the Sigma 28-70 or 24-70 (both EX)EX, the tamron 28-75 f/2.8, and the nikon 35-70. These are all alternatives from the nikon 28-70 f/2.8 since the price of that lens is so high. Pretty much, i'm asking "Which one of these lenses are closest to the Nikon 28-70 f/2.8?" I am a portrait/wedding photographer, as well as nature. So I aquire a sharp lens. Any picture comparisons would help tremendlously.