Thanks for that excellent comparison!
I think what I'm trying to do, is justify to myself that I should spend the money on a Shield. They're cheap right now, places have them on sale, and to top that off there's the $75 S&W rebate.
But I don't NEED another small carry gun. Really, I'm perfectly happy with my PF9. Shield triggers are far better though, and I hit better with them - at least the 9mm ones I've shot.
So, to make this potential Shield purchase more distinct and not a near-duplicate of things I already have, I was thinking of either (1) a Shield in .45, or (2) a 9mm Shield with that green Crimson Trace laser on it. Just a standard 9mm Shield is quite close to my current PF9. Better trigger, slightly larger, heavier, won't fit in a pocket (the PF9 does - barely) and I'd need to buy a new holster for the Shield. I like custom leather ones, so that gets pricey. I'm not a fan of IWB carry, so I would go OWB with a cover garment for a Shield.
When it comes to .40 vs. .45, I generally like .45 better. The .45 recoil seems a tad heavier to me, but it's more of a push whereas the .40 is a snap. I can control the push better. However, I shot a full size M&P in .40 and that was a very nice gun. I shot extremely well with it - I surprised myself with how well I seemed to control the snap. Other .40's I've shot though, I didn't care for the snap as much as with that full size M&P. But for a Shield, I'm only considering the .45 and the 9mm at this point.
In a similarly purposed gun, we have both a Springfield XD/S .45 and an XD/S 9mm. Actually they're both the wifes guns. I like the 9mm better of those two, but the .45 is not bad. A bit more recoil, but not a large amount more. Both are easily controllable. The 9mm XD/S is more fun to shoot for long periods. We also have a full size XD/M in .45. With a double stack mag, that thing is as large and heavy as a school bus. You would think it would kick a lot less than the XD/S .45, but it feels about the same to me. Possibly because my hands are small and I can get a better grip on the single stack XD/S. A small gun feels more natural in my hands than a large one.
My son has a Shield in 9mm. I've shot it a lot, and I like it better than the XD/S. But that is personal preference. My wife likes the XD/S better than the Shield. To each his own!