AdrenalineJunky, I carry an issued M&P40C (compact version in .40). It fits nicely in my hands, conceals very well and shoots flawlessly. My dept. went with them back in the winter/spring of 2008 and the vast majority of our 1,700+ officer dept. seems to love the pistol. I personally love the interchangeable backstraps so that you can tailor the grip size to what feels good which is very important in a .40 where recoil can be kind of snappy. Especially if this will be your first gun, a .40 can really bark in your hand. I also like the M&P's trigger and it has smoothed out quite well with dry firing and a few (undisclosed number of) rounds down the barrel in practice and qualification.

I have not shot an SD but did handle one at the NRA convention. It felt pretty good in the hand but I definitely prefer the M&P. On a side note, I have a co-worker that is also a collector. He's got everything from older Smith's to Browning’s, Colt's, H&K's, Kimber's, L.C. Smith's, Parker and Hale, you name it and he's either got it or had one. He carries the full size M&P40 at work and has been a fan. So much that he bought an M&P45 and said it was his favorite shooting .45. This coming from a guy who had several Kimbers that were $1,000+ EACH! He also picked up the M&P9 PRO which is a long slide and barrel target version of the full size M&P in 9mm. That pistol has a custom trigger on it and a fiber optic front sight and black rear sight with no dots. I love the trigger on that pistol and the first two rounds I shot with it went into the same hole. I quit after that, knowing that I wouldn't do any better.

If you have an opportunity to shoot either of them before buying that would probably help with your choice as well. Check to see if any of your local ranges or stores have M&P and SD rental guns or a Smith and Wesson Demo Day coming up.

Good luck with your decision!