I put three X3's on last year. One on a stock 2013 Pro, one on a Stage 4 SLP Pro, and one on my BD turbo Pro. All three were 163's. We ride Eastern Idaho/Western Wyoming and in our soft snow, the 3" was amazing. They do require more feedback when sidehilling in the tight stuff. My buddy on the stocker is a little guy and he had to work harder in the tree's. My buddy on the Stage 4 is 6'3, 220 lbs and athletic. He loved his. I'm 6'2, 200 lbs and athletic and I wouldn't change a thing.
I especially like the 3" on the turbo. I personally think the 3" loads the turbo more, which I prefer.
I have no experience with the 2.5". I'd be willing to bet that coastal snow conditions would be better suited for the 2.5"