I thought about doing this, but ultimately went with a Kel-Tec PF-9 for pocket carry. My pocket holster holds the pistol and a spare mag, so I have 15 rounds of 9mm on hand.
I believe your entire kit will be interchangeable with both .40 and 9mm M&Ps. I think the reason they asked you is because they sell a different kit for the .45 ACP models. When they first came out with the Apex sear for the M&Ps, they found that it wouldn't work right in some .45 ACP model...
Many if not all M&Ps will do this. It's just caused by the slide release jarring lose when someone puts a magazine in hard. Some M&Ps will only do it if you insert the magazine up and forward at the same time. I usually try not to insert my magazines so hard that this happens. Some see it as...
A local pawn shop that has a great selection had a SD40 in yesterday. I liked the way it felt in the hand, but the trigger pull was a little heavy for my liking. They were selling it for $380 I think.
Here's the official page from S&W, they just posted this on their facebook page, saying the SD9 and SD40 are supposed to fit between the Sigma and M&P series pistols: Introducing the SD9 & SD40 - Smith & Wesson
I can't speak for the Sigma, but the M&P 9mm is a great handgun. I'd call that a pretty good price for it, considering that it has nightsights and the laser/light combo. The triggers aren't the best on the M&P, but they do break in a bit if you get lots of trigger time (dry firing with snap...
Mine doesn't hit me in the face, but almost every piece of brass flies right over the barrier on my right and into the next lane at the range. I usually try to not start firing it if someone is mid-magazine in the lane next to me. Then again, they could use it as training for distractions!
Like Lee said above, the stock sight is a bit difficult to remove. I'm glad I wrapped my punch in masking tape, because it took several hard hits with a claw hammer just to get it started. Once out, the install was painless and getting the sight back in place wasn't too bad.