Take her to the gun store, have her hold each and get familiar with the controls, and let her pick.
And try to find one with a rental range so she can try them out. That is what my wife wanted to do. Keep in mind my wife purse carries with a nylon haversack, so the size of the pistol isn't as much an issue. She is also real recoil sensitive regarding handguns in particular, and when we first started to outfit her, her idea of the ideal weapon was light, small, with no recoil and in a sufficient defense cailbre. It took awhile, but I finally convinced her no such weapon exists. Getting that out of the way and getting her to accept a heavier piece that she was comfortable and good with was the hardest part.
As far as Glock vs. M&P, well, again she needs to feel them out, better yet, shoot them both. They are both fine pistols, but I like Glocks. The tennifer treatment can't be beat, they work very reliably, 3 safeties --none of which you have to fumble with, it is just a no-nonsense pistol. I like the polygonal forged barrel too. I don't shoot lead, and those barrels are seriously underestimated.
But I have a lot of Glocks. Some would call me a "Glock Guy". I do like my Smiths, but they are all/have been revolvers or 3rd gen. autos. So I had a lot to let my wife try the Glocks. The speed with which the weapon can be put into action is a great plus, as is the consistent trigger. Maybe not the best trigger, but it always feels the same. So I let her try mine, a G17, G27, G29, G17L, and she hasn't shot the G20 or G21 yet (out for repair and no ammo loaded yet respectively). Believe it or not, out of all of those, she thought the G27 followed by the G17 with 115gr.+P to be most offensive. She actually liked the G29 with 200gr. XTP @ 1050fps! Go figure. Larger pistol with a "light" load though... Kind of expected. She did not like the 155gr. or 180gr. XTP at any loadings. She likes firing the Beretta 92, but dislikes the trigger. Doesn't like enough to carry.
So she will most likely end up going with the Smith 10mm auto. Big and heavy, but she likes the recoil and the round is sufficient (200gr. XTP @ 1050fps). She is accurate with it too. Keep in mind any polymer frame pistol will change how the recoil is perceived. Feels more "snappy" to some. The heavier slower bullets "shove", have a lower impulse, so the recoil isn't as brash.
The other candidate ended up being the Sig P239 (I think). The one with the single stack .45ACP. Standard model. Again, big heavy slow bullet with a large metal frame and heavy slide.
I would have sworn the 9mm would have been the way to go with her, but she only likes shooting 9mm practice loads that I make, 124gr. plated @ 1100fps. from the G17L.
When shopping for your wife's pistol, make sure to let her fire a few candidates. Her likes might surprise you. Oh yeah, and don't discount the stainless .357 revolver, say a snub 686+. My wife LOVES our 686+, but it has a 6"bbl. No doodads either, just pull and shoot.
Another fellow on here said that everytime when shopping for a weapon for a woman, they will surprise you at some point along the way. My wife obviously surprised me, choosing 10mm over 9mm. But I've seen a 16yo girl, my buddy's daughter, do the same thing --said the G27 was too brash but she shot all my 10mm loads! Then again, 9mm is popular amongst women too...
The absolute worst thing you can do is saddle her with something she won't use. Some women will abdicate the decision to the husband in order to please him and end up with something like this... My wife is prone to that so I have to make sure it is exactly what she wants and will use.