Has your wife (or have you) fired any similarly-sized .40 caliber handguns?
Here's a little story that might help you decide on the Shield .40. My brother was very enthralled with them, also. He was dead set on finding one (they seem to be in short supply) and purchasing it for his CC pistol. My brother has very limited experience with handguns so we went up to our farm and I brought along a few different pistols for him to try. I brought 9mm and .40 caliber pistols, including a Glock 19 and a Glock 23 (essentially the same pistol in 9mm and .40 caliber, respectively, if you're not familiar with them). I had him shoot the Glock 23 first - simulating a rapid-fire defensive scenario - and then the Glock 19. With a paper plate-sized target at about 7 yards, he had one shot on target with the G23 (.40 cal) and four shots on target with the G19 (9mm). He was quickly convinced that small .40 caliber pistols were much more difficult to handle in rapid-fire scenarios (typical defensive shooting).
I'm not suggesting that the Shield .40 is a bad gun. I've never fired one. However, the Shield .40 is considerably smaller than the Glock 23 and, by extension, even more difficult to handle in real-world defensive scenarios.
I know many women are tempted to carry very small pistols due to the lighter weight and how they fit in their hands. That's not necessarily a great way to choose a defensive weapon.
