One of my favorite baselayers!
The capilene 3 have served me well for the past few seasons. It works great if the temps are between 5-30 degrees. If it's a particularly cold day, the capilene 4 is the way to go, but for your average day on the hill, these will work really well!
I also wear these under a softshell from XC skiing. For the more active XC skier, I'd suggest the capliene 2 since you'll be moving around a bit more... but if you're just doin some classic skiing, the 3 will be perfect!
Compared to a midweight wool baselayer, they perform about the same, although the wool will feel a little lighter.