I am, I guess, a bit of a Magpul fanboy. So, unsurprisingly, I went with Magpul's PRS (Precision Rifle Stock) butt-stock to replace the factory adjustable butt-stock. I prefer the PRS over the factory adjustable butt-stock.
I changed out the receiver extension (also called buffer tube), buffer, and spring. I used the Aero Precision M5 Riffle Kit, which includes a rifle length receiver extension, a riffle buffer (heavier at 5.1 oz vs the 2.9 oz of the factory carbine buffer), and a rifle length spring.
After some initial trouble with SOME ammo, my initial thoughts were that I did not hit on the right combo for the butt-stock. Turns out, it was a lube issue with the bolt, not the replacement parts. Don't let this "proprietary" talk put you on unsure footing like I did. No need for a spacer when installing proper rifle length components.
My M&P 10 shoots really nice - I am the component that needs to be improved, and the soft recoil the gun now has will enable me to isolate, and improve upon, my short comings a little bit easier.
Again, the Aero Precision M5 Rifle Buffer Kit, was all I needed.