I have a Yukon smock and I've taken a close look at a Plasma but don't own one.
Plasma :
+ better hood design
+ eVent outer
+ alpine / mountain cut (less boxy shaped and somewhat longer in the body than the Yukon)
Yukon :
+ choice of materials
+ lighter weight
+ somewhat better down, especially if you take the 900fp option.
+ fewer distribution problems as made in and shipped from UK
The Yukon is definitely not designed as an alpine climbing jacket where the Plasma decidedly is. But if you just need a light weight down insulator the Yukon is a great piece to own and you'll save a significant amount of weight.
Just to confuse things, if I was in the market for a lightweight box wall down jacket I'd also take a close look at the Valandre Split S. I have the looping and it's an excellent garment, beautifully made and well designed. There are many other options but those are the ones I have some experience with.
John