A full face balaclava is your best bet when you're dealing with seriously cold conditions or biting wind. It covers your entire head, neck, and face with just an opening for your eyes (and sometimes nose), giving you maximum protection from the elements. If you're riding in single-digit temps, facing wind chill on exposed lifts, or spending long days in the backcountry, this style keeps your skin sealed off from frostbite risks.
The trade-off is that all that coverage can work against you in certain ways. Full face styles are the most likely to cause goggle fogging since your breath has limited escape routes, and talking to your crew on the lift requires pulling the fabric down. If you run hot or ski hard enough to break a sweat, you may find yourself overheating.
Save the full face option for the coldest, windiest days or high-exposure missions. For average resort days or high-output touring, a half face or convertible style will keep you more comfortable overall.