There were many reasons for my switch, and if you’re curious you can flip through my article archives. What follows isn’t a reason for my switch, but I think works out to being a good but subtle win for the M&P over the XD.
First, in most respects the guns are the same and both will function fine for most people. Both strive to provide some degree of ambidextrousness, but neither are 100% ambi. The XD has a mag release button that works on both sides, but the slide-stop lever only exists on the left side. The M&P has a single-sided mag release button that with a little bit of work can be flipped over to work from the other side, but it only truly works from one side or the other. However, the M&P does have a slide-stop lever on both sides.
After my prior “weak-hand-only” dry fire session, I noticed how much my old XD habits remained and that I hadn’t yet created the muscle memory for the M&P in this regard. It didn’t take me long to adjust to using the middle finger of my left hand to press the “same-side” mag release button, but even by the end of the session I was still forgetting about the slide-stop on the right side. But it made me realize something:
I think the M&P has a better ambi setup than the XD.
First, for a left-handed shooter, the M&P wins because you can have all controls “in the normal positions” on the M&P. A little work to flip the mag release button, and you’re there. Nothing can be done with the XD to make it 100% lefty-friendly like this, tho you can work around it.
Second, for someone like me where left-hand is “weak-/support-hand”, I still think the M&P wins because if I still have enough hand to shoot, then I still have enough finger that can press the “same-side” mag release button. And then all other controls are where they should be. But as witnessed by my dry session, you have to learn it because most guns are not set up with slide-stops on the right side and so you have to make sure you train your body to use it. With the XD, you just have to train using other approaches.
So all training issues aside and looking purely at the design and mechanics of the guns, I do think the M&P gains a slide edge here.