A convertible balaclava is a great choice if your day involves shifting conditions. It has a hinged or detachable lower section that you can pull up for cold, windy lift rides and drop down when you're working hard on descents or warming up in the lodge. That adjustability means you're less likely to overheat or fog up your goggles compared to a fixed full-face design.
The trade-off is that convertible styles can be slightly bulkier at the hinge or attachment points, and when the face section is pulled down, there's potential for small gaps that let cold air in. They also don't seal quite as tightly as a one-piece full-face balaclava in extreme conditions.
If you typically ski in consistently bitter cold or high wind, a dedicated full-face model may be warmer. But for resort days where temps fluctuate or you're mixing casual runs with high-output skiing, the versatility of a convertible is hard to beat.