I can't even understand why the comparison comes up, really. The Shield is hugely bigger than the PM9, and when it comes to pocket carry, every inch and ounce makes a gargantuan difference.
Here's a great, visibly unbiased review of both guns side by side:
And here's a review of the Shield vs the 9C:
I was seriously considering a Shield, except for a few reasons:
1.) It's much bigger than the PM9
2.) That safety is far too small in a SHTF situation, and the trigger is far too short for me to pocket carry without SOME worry without it engaged. The PM9 is the sweet spot for a trigger with no need for a safety
That said, the Shield is going to sell like hotcakes. It's a great "small" pistol, but I don't think it's small enough to be in the sub-compact market (just my opinion). But the price of the Shield is FANTASTIC, and the support for sights/holsters out of the gate is fantastic. Let's not forget the Shield will come with a LIFETIME WARRANTY. That just flat out rocks.
Also, the Shield is like the other M&Ps, so the takedown procedure is sooooo easy by comparison to the PM9, which - let's be honest - can be a tad fiddly: Line up the notches, stab at the slide-release lever. Fiddly recoil spring, having to push down on the barrel to reassemble etc. Not exactly simple. Sure, you get used to it, but the M&P line is a DREAM to take down and put together. Probably the easiest gun to take apart that I've ever used.
The more competition the better, but I think people who hold both the Shield and the PM9 will pick the PM9 for POCKET carry. For IWB holsters, the Shield may get the nod if you like the other aspects of the gun. Also, it's an M&P/S&W, which let's be honest - is going to sell better than the Kahr just in name alone.
THAT said, it's crazy how much more expensive the PM9 is! I paid, what... $750 including tax for my PM9/stainless w/ night sights + $200-ish for the Crimson Trace Grips. That's nutty...
For reference, I also owned a Sig P290 for a while, and it was huuuuge compared to the PM9 in my pocket. And yet the P290 is considered a sub-compact, too.
Different strokes, right?
PS I used to own an M&P 9 Pro Series, and didn't like the trigger, so I sold it. Too notchy.