The sig 105mm has a ridiculous filter size of like 105mm or something from what I've heard. Perhaps this is just like their 84.6 errr I mean 85mm Art, meant to counter vignetting wide open. The 105 f/1.4E has ample vignetting wide open. CA is also quite moderate, although I'm not sure if it's an issue in shooting or not -- I don't have one. Maybe the Sig has better performance when it comes to lack of CA?
As for the 70mm, I'm a serious macro shooter, not interested. I have an 85mm pc-e for 1:2, a rayfact 95mm f/4 that covers 1:1 to 2:1 (it's not on Nikon's website, it's a special line scanner lens that covers medium format). I would like to have 1:1 macro lens that gives me adequate working distance, which the 70mm would not deliver. Instead, I'd take the 180mm from Sigma. Going to save up for one in the near future.