Soemone asked about SR9 vs SD9- I have the SR9c, and the SD9, and I gotta say, I love em both. I carry the 9c, with the thumb safety, and keep the SD by the bed, with the Micro90 light on it. The trigger is probabaly a tad smoother on the SD, but a tad shorter in the 9c. But both are reliable, and easy to shoot, and accurate.
The SD actually has quite afew improvements over the Sigma, thus the extra hundred or so bucks. And I understand the M&P has a few improvements over the SD, thus the OTHER hundred bucks. I would buy the Ruger or SD again without hesitation.