Held off on posting in this thread because of it's age but...
A major selling point to me regarding the M&P .40 is that you can swap out the .40 barrel and replace with a .357 barrel, keep the existing .40 mags and suddenly, you have a M&P .357. Or you can swap out the .40 barrel and replace with a 9mm barrel, purchase 9mm mags and again, suddenly you have a M&P 9mm. You cannot do this with any of the other M&P handguns. S&W with not sell you just the barrel but you can get after market barrels.
As for the "gritty" trigger pull, it's been stated before and I will confirm, this is only an issue when the firearm is new. After firing a few hundred rounds downrange and cleaning it a few times, the "gritty" feel goes away.
As for after market parts, I have the CT grip mounted laser sight and after some practice, it can be removed and reinstalled while the firearm is still in my Blackhawk Serpa holster in less then 30 seconds, one handed and without even looking at the firearm. The CT grip mounted laser sight is about mid range between the medium and large stock backstraps and seems to fit my hand just fine.
I did a great deal of research before purchasing my M&P .40 and it was ultimately the one I decided to go with. It has a nice heft to it, feeds most types of ammo with no issues, breaks down very easily and safely (with the seer release lever), is easy and simple to maintain and clean, and all around just feels like a quality firearm for daily carry. These are all important things to take into account when it is your every day carry and has to be reliable every single time.
My roommate has an SD 9mm and I can't stand the thing. It feels cheap, is a pain to break down for maintenance, and just does not have anywhere close to the same features as the M&Ps.