A half-zip mock neck is the most versatile option for skiing, especially if you ride in changing conditions or tend to run hot. The short zipper (usually 6–10 inches) lets you dump heat quickly when you're working hard—like hiking to fresh tracks or skiing bumps—then seal back up for cold lift rides.
The mock neck collar adds a bit more coverage than a crew neck, which helps seal out drafts around your throat. This is a real advantage on windy days or when you're not wearing a neck gaiter. The trade-off is that the zipper creates a slight bulge under mid-layers, though most riders barely notice it once they're fully layered up.
If you ski backcountry or deal with variable mountain weather, the half-zip is hard to beat. A crew neck works fine if you always wear a gaiter and don't mind giving up adjustable venting, but most skiers end up preferring the zip once they try it.