There is no doubt that mittens on average are warmer than gloves. They allow four fingers to use each other for warmth on the coldest days. Children who do not rely on poles for skiing will find mittens most comfortable to maintain warmth on cold days, as will snowboarders and other skiers who are forgoing poles.
Mittens can be made of the same materials as gloves, hover, due to their unique shape, they allow more warmth to be maintained on even the coldest days. Mittens may even be often part of the wardrobe of professional skiers, who may have no problem gripping a pole with their hands enclosed by the mitten shape.