Look at these holster makers' websites bec they not only make some of the best holsters around, but they also describe what makes a good holster and why. So, even if you don't buy from them you will learn a lot:
Garrity's Gunleather
5 Shot Leather
Mitch Rosen

carrying a gun is a lifestyle commitment, so you will have to adjust your wardrobe to make it work. The best way to conceal a large gun is IWB. Personally, I hate carrying large guns IWB and, therefore, I carry nothing bigger than a Kahr K9 Elite 03 or a S&W J-frame IWB.
However, I carry large guns OWB and these are not only very comfortable, but also very well hidden as long as you dress accordingly. You must have a high-riding pancake/paddle holster or the tunnel/loop holster to hide it well. That's why you should look at the websites above bec they discuss all this very well. You will learn.

I highly recommend the Kahr K9 Elite 03 - 7 or 8 rounds of 9mm in an all SS package for IWB - or BUG. They make Polymer versions if you prefer that instead. But once you shoot the K9 Elite you will want it. It is striker fired like the S&W and the SA XD. I would then carry your M&P OWB.

One last option - Shoulder Holster? Look at Garrity's SH - pretty much the best.