Welcome to the forum!

I think they are both great pistols, but I would personally go with the SD9. I have owned my SW40VE for well over a year now and I do love it. But I had the opportunity to spend some quality time with a friend's SD9 and it's very nice. I prefer the melonite finish on the slide and the factory installed front night sight is a nice touch. The standard accesories rail on the SD series is also a plus compared to the unique design on the Sigma which requires an adapter. In my opinion, the SD9 is the bridge between the Sigma line and the M&P. It seems as if S&W took cues from both and merged them together. Just some food for thought.