Paul: Those numbers don't mean much. Maybe OK for comparative purposes, but I wouldn't take them literally. If you plan to sit for any length of time, neither will keep your feet warm at minus 20 degrees fahrenheit. Both will keep them comfortable if you're walking.
Actually, my feet tend to sweat in them if I do a fair amount of walking, regardless of the temps, but I'm seldom out in temps colder than minus 10 or so.
Either boot will likely fine at the moderate temps you're likely to experience on your stomping grounds, say 20 degrees, maybe a bit colder at times.