There are plenty of considerations before you can choose which mittens are good for you. Look at the:
Size: When you buy any piece of cloth, you want to be sure it fits you well. Same goes for mittens. If you choose an oversized pair, it won't keep you warm. When they are too small, you won't be comfortable wearing them. Your movement will be limited if you manage to slip your hands in them. Measure so that the glove feels comfortable when you fold a fist and still fit snugly when the hand is free.
Additional Features: If you'll be using your phone while wearing the mittens, you want one that has touch-screen compatibility feature to avoid removing the glove every time you want to access the device. Some come with zipped pockets where you can store your keys or other small items. Others will have multiple adjustable straps for a custom fit. Look for gloves with hooks you can use to hang during storage.
Waterproofness: You want your mittens to be breathable and still keep water from reaching your fingers even when it rains. The material used should also wick sweat so you don't drench your hand as you have fun.
Insulation: There are two types of insulation used in mittens; down and synthetic. Down insulation works great for activities like snowboarding, skiing, and in dry environments but are costlier. Synthetic insulation works better in cross-country and downhill ski.