Differences between the SD and the Sigma:
SD has steel M&P sites, front one being a nite site
SD has a standard rail for attachments
SD comes w/ better, lighter trigger than Sigma
SD comes in stainless w/ black melonite coating

For the amount of money difference between the two, my opinion is go for the SD! They have improved most of the issues that people have with the Sigma. The steel sites on the SD are a big enough improvement over the blocky plastic Sigma sites to justify the price difference to me.
P.S. I have both, and neither of them have ever jammed or malfunctioned.