Tires!
Let us first acknowledge that the WS-70 holds an edge over the Xi2 on ice.
Let us also acknowledge that the WS-70 holds an edge over the Xi2 in snow, especially deeper snow.
However, on clear roads, dry roads, et cetera, the Xi2 will be quieter, better handling, and generally feel more like an all season tire than the WS-70.
I do not like that the Xi2 is "Green X", Michelin's low rolling resistance, because low rolling resistance has only one real drive - you sacrifice traction.
To me, a minor increase in fuel efficiency is not worth a minor decrease in traction. In fact, a moderate increase in fuel efficiency is not worth a minor decrease in traction.