I prefer gloves over mittens. I could never get comfortable with the "trigger finger" idea but my buddy swears by it.
I used to ride with mittens because it was the "cool snowboarder" fad. I have since realized that life is much easier with gloves (strapping, unstrapping, adjusting gear/goggles, etc) and never went back to mittens. We ride in single digits with very cold wind chills and my gloves perform perfectly well.