My wife and I carry M&Ps in 40, full size and compact. It is the same size frame as the 9mm. M&Ps are great. The biggest downside is the trigger. I find the reset to be very smooth and difficult to "feel." We carry in OWB holsters (Dragon Leatherworks Flat Jacks) and have no issues. The mags interchange with the full-size which for me is a big plus. The Apex RAM kit addresses the trigger issue if you want to spend a hundred bucks or so.
The Shield is thinner and has a better trigger. I've also heard it is very shootable for a single stack slim frame pocket nine. Single stack means less capacity, though.
I think they fit different roles. The M&P full-size is a serious fighting service handgun, ideal for open carry, CCW if you're a bigger person, or a defensive "house gun." The M&P compact is its more concealable counterpart; despite a shorter sight radius, you've still got a good grip, good capacity, and ability to carry a full size mag for a 17 rd reload. The shield is a hold-out gun, more comparable to a pocket 380 or a 38 snubbie. Most civilians are going to carry at most one spare mag, so with the shield you've got 14+1 rounds available, and with a M&P compact you'll have 30.
Pistols are all ballistically inefficient and a minimum standard response should be more than one round fired per target. A realistic "most dangerous" threat scenario is multiple threats (say, 2-3 thugs). 7+1 in a shield feels a bit underwhelming when you consider a scenario of two or three threats who may need two to four shots to stop as a minimum standard response. I think the shield is a well-executed pocket gun and I think it likely stacks up very well if compared to a Kel Tec 380 or a 38 sp snubbie but for me, I stick with the M&P compact.