Let me add another one to the mix here. I was also taking into consideration the S&W M&P .40. My brother in law is a cop and hates double action only pistols and says that if I buy this gun I will be making a huge mistake. He likes double then single. I dont have enough experience to tell the difference but would like your oppinon.
The glock is a DAO too. So if you like that, you don't have an issue with DAO like he does. I am not a fan of it either, especially ones without exposed hammers. The whole reason for DAO without safeties and without hammers is to make a gun that when you pull the trigger, it goes bang with no fuss, no muss. When you have cops with varying levels of talent and comfort with guns/shooting, making things as easy as possible is a highly desirable trait. The down side in DAO is every round has the same trigger pull, and you can't have a really light trigger pull ever because you need to cock the hammer (hammerless don't suffer this problem as badly as mechanical hammered guns do). DAK was made to address this to some extent by getting a pull that is similar to a SA trigger weight.
So, what he likes is DA/SA where the first round is double and a stronger, longer pull while all subsequent rounds are single. The nice thing here is a gun without a safety has a much harder trigger pull for a very long travel (the 229 needs to be pulled about ~1/3" and at ~10 pounds of pressure in DA) on that first round, and is therefore more "safe". Also, with DAO and DAK that trigger travel is always long. I don't care for long trigger travel and many, like your friend don't either. In SA, the trigger travel is ~1/8" and the pull weight is ~6 pounds on most handguns, give or take. So you get the relative safety and simplicity of DA on that first shot, but then you get the accuracy and performance of SA on subsequent rounds.
There are also guns like the H&K USP where they have DA/SA with an actual safety (in variants 1-4), but again this complicates things for inexperienced shooters or shooters in stress conditions.
The glock and other DAO are safe too if you keep your hand off the trigger, then fine but sometimes things other than hands/finger touch the trigger. Get a VERY GOOD holster with DAO like a glock and DAK because the trigger pull is much lighter than 10 lbs in order to make it shootable and accurate. Shoot the sig DA and you will know what I mean. The pull is so strong that you will invariably shoot down and to the left because of the force needed to cock the hammer.
Hope that helps explain what the differences are and why they are that way.