Welcome to OCDO!!!! Stick around and you will learn a lot!!!!
You should check out the Maine forum here.
As for the SD, my understanding is that it is the "economy" model pistol. I believe it is made using cheaper parts and less machining. This lowers the price point to make it more appealing than the M&P as an entry level pistol. The SD is also an improved model of the poor performing Sigma line and incorporates some M&P features. It is essentially a hybrid of the Sigma and M&P. Can someone back me up on this or am I way off?