Let me see how short I can make this.
In Alaska where temps were around 20 F in winter I wore wool mittens. No gloves seemed to work well. I had wp nylon mitten shells in my pack that i rarely used over them. I have experimented with all sorts of gloves, wool, synthetic, leather/fleece/nylon rei combo and others. They all have their place. (handling cold ice axe for example). I have tried and do like the flexibility of the glove/mitten hybrid. These days, in winter, I always start out wearing gloves, but keep a pair of O.R. mittens (gortex shell, fleece liner) as a back up for colder conditions. That said, a pair of cheap ragg wool mittens is perhaps the lightest, warmest, most practical thing you can carry. Don't forget you extra wool socks can double as mittens in an emergency!
http://www.campmor.com/outdoor/gear/Product___92881?CS_003=2477120&CS_010=92881