I posted my thoughts on page 1 of this thread but will add a few here, if you folks don't mind.
You best friend in taking any decision is to exercise the clients. In other words, go to a range and shoot them. Preferably with a friend who owns guns which you are considering. Next best thing is renting at the range if that is an option.
We can look at the history and reputation of given defensive handguns and glean a measure of factual data as to the trustworthiness of your selection. And this is a very good thing to do. The next thing is to examine the guns very carefully, looking at things that many folks overlook or don't thing about when making a purchase.
How hard is it to "rack" the slide? How hard is it to release the slide stop? How hard is it to load the magazines (compare the Glock to the M&P in 9mm and you'll know what I mean with this one)? What about the shape of the trigger guard? How hard is it to detail strip? Are replacement parts readily available and can you install them yourself? What is the shape, width, and angle of the feed ramp? Is the magazine release easy to use yet secure from accidental activation? Does the bore axis height work for you? Are the sights good (can you obtain a sight acquisition and a sight picture reasonably quick)? How hard is the gun going to be to clean?
There are a host of other questions and points to consider when deciding upon a defensive firearm, but all of this pales when weighed against the one primary and major concern: does it go bang when it is suppose to? The loudest sound in the world in an extreme encounter is "click". Reliability is paramount and supersedes all other considerations.