I would say the reach is the main difference between the 2 lenses: they are both excellent. I loved my 28-70d f/3.5-4.5, a lot--here is 1 from it, one very cold, October sunrise in Vermont.
I've shot with a lot of Macro glass--get the Sigma 150. I've put a lot of shots with it in this thread here: The Macro Club | Page 50 | Nikon Cafe there are many other macro lenses in this thread too, including the Sigma 105--this shot is not a crop--w the Sigma 150mm
Agreed--it looks like a 24mm in the images. The Nikkor Ai-s 28mm f/2.8 is a great lens for close-focus, where it is optimized, although is is decent 2 infinity too. Here is one with the 28mm Ai-s on the D700.