I definitely prefer the Vive DAS over the SMAS.
The speakers on the SMAS sound awful (think like turning an AM radio on in the background), don’t isolate you at all from outside audio, and blast audio into the open environment.
The MAS headstrap itself feels cheap in build quality and does not hold the headset securely enough on my head without the headset shifting out of the sweet spot when I turn my head even slightly during use, no matter how far up or down my head the headstrap is positioned, or how much I can tighten the headstrap. The instability is due to a huge gap between the side of my head and the side of the headstrap which is large enough for me to stick my whole hand through, so it has no support at all from the sides to keep the headset secured. With that aside, the headstrap doesn’t even feel more comfortable compared to the Vive DAS with all of its added complexity.
I was excited to eventually buy the KDMAS later after backers received theirs since I opted out of stretchgoals, however after trying out the SMAS I have no plans to get the KDMAS now.