in exchange for not much warmth benefit. Keeping your hands moving will produce enough heat in gloves to offset the mittens benefit.
Ive never found that to be the case. My hands are nothing but cold in gloves, no matter what you do. They also suck if you need fine dexterity, like handling/manipulating anything small, so you still have to take them off too. With mittens, the difference is instantly noticeable, and thats even with taking them off constantly, if you need to do fine work. If Im the least bit active with mittens on, my hands arent even chilly.
If you find you need a little more dexterity, there are always the trigger finger models that give you a finger (which you dont have to use if not needed), or the hunting versions, with the slit, that let you use all your fingers, but they tend to be not as warm, and those slits dont usually stay closed well.
The mittens with the flap and fingers/half fingers, are only "slightly" better than gloves, as they keep the fingers isolated from each other, and dont allow meat on meat warmth.
I use a couple of pairs of surplus military mittens for work, as they are cheap and tend to hold up to constant work in bad conditions better. They usually arent as warm as some of the commercial mittens though.
If you want warm, really warm, a good pair with a waterproof/windproof outer shell, and removable heavy fleece liner, will keep your hands toasty.
One other advantage to mittens is, you can store stuff your using in them, (small tools, parts, etc) and you can also keep some of those heat packs in them, if youre really cold blooded. I cant do it, makes them "sweaty" hot.