I agree, stick with the M&P. If you've got it tricked out for CCW or comp shooting, get another one and slap a day-glow sight on it. Keep that as a duty gun. Break it in, keep it respectfuly clean, but don't worry about beating it up. The gun can take it.
You need to have a gun you know the feel of. Everything should be instinctual, from the draw to the reload. You could choose to learn a revolver for that purpose, but you're looking at some time for that practice to become training. During this time, you are at risk from your own skills. If the M&P is approved, and you're good with it, stick to it. Be sure to practice Failure to Stop drills. Most handgun calibers don't have the velocity to punch through body armor.
The important thing is to remember is that if you're attacked, you'll probably be behind the curve, and you'll need to be thinking of everything except how to draw and shoot the gun.
An America's Most Wanted a few months back has a reenactment of an armored car person being attacked outside a theater. The deliveryman had no chance to react as he was shot from behind. There were all kinds of failures of the system to result in this, most notable being (at least according to the reenactment) the driver not noticing a guy with a hood and sunglasses hovering in an alley near the door. The Deliveryman wasn't looking around for threats, checking six, etc.
Had the threat been spotted fast enough, the guy could have deployed his weapon. Had the driver recognised a possible threat, he could have waved off the return trip for a few minutes.
Point is, your weapon is adequate for the job, and you have the practice with it already. Refine that skill, rather than spend a lot of time learning a new one.
. I've had the metal part get caught up on the belt loops of my uniform pants or BDU shirt more than a few times.
