Since 2018 I´ve been riding doos with t-motion and ones with rear arm locked. Personally I like my rear arm locked, but can understand riders who do not.
Deeper and softer the snow is, more you can benefit from the t-motion. Less effort on carvin, faster and lighter manuverability. In other words, more track speed you can carry while riding, t-motion claimed unstability is not gonna be that much of an issue. But on hard packed snow, and when there is layers, you might get problems holding the line. Old tracks and changes on snow pack can throw the sled on unexpected directions. That doesnt give you the confidence you might need when going full speed while skis are scratching the trees.
If you ride 165" or even 174", t-motion definately makes the sled more agile and faster to manouver. Negative qualities like unstability are not that much of a concern as with 154" or even 146". On 146" t-motion is definately an overkill as track lenght isnt enough for deepest and steepest conditions anyway. With 154", it really comes to a riders prerefence, riding style and conditions if you should have it locked or not.
Here where we ride, conditions change a lot during the season. With locked rear arm, I feel the sled is not that sensitive for those snow pack changes. I can trust that sled will allways behaves the same way.
But its great that Doo has options nowdays. You can tune your sled either way you want it as there is official option for locking or unlocking the rear arm. After all its a personal preference.
One really interesting thing for 2023 is the new Pilot DS-4 ski on expert. Old DS-3 has it problems, especially when there are layers on snow pack. Seems to be that new ski has more straight profile, as old DS-3 is a lot more curved. Should be more stabile even on hard packed snow and especially on higher speeds. Seems to be that DS-4 is something between DS-3 and Lynx Blade DS+ ski, but hard to tell before I get some miles on real conditions. We will see if its gonna be a stantard for other deep snow lineup too for 2024.