I have the Sigma and am very happy with it, the build quality is good and the imaging quality is excellent. I am holding out for the Nikon VRII 105mm Micro.
In answer to your question, all things being equal I'd probably go for the Nikon. I needed a macro for a shoot and it was the only lens of quality I could get my hands on, I haven't replaced it yet as the quality of the final product gives me no complaints.