the idea is a skinnier tire penetrates through the snow cover. this is great if the snow isn't very deep. a larger tire floats more, and with larger lugs will get better traction in the snow. if the snow is fairly wet or compacted, you'll want studs as well for the inevitable ice. there are many competent "snow" tires, there is no such thing as an "ice" tire; a skinny tire is just as good/bad on black ice as a wide tire, and debateably worse in heavy snow over black ice, as the wide tire will float whereas the skinny tire will penetrate and slide sooner.
it all comes down to climate. i grew up in alaska (18+ years there) driving in heavy snow and black ice 6-7 months out of the year, and studded snow tires are a must. in colorado, studded snow tires are rarely necessary unless you spend a great deal of time at high altitude looking for snow.
your handling will not improve...skinny tires "wander" more than wide tires, but will get better fuel economy. 245/70 is what base-model tundras come with anyway, i wouldn't sweat the speedometer.
uh...i guess that was a lotta text, but basically if you're in snow, like deep fluffy fresh snow, you want a wide tire with large lugs (
www.arctictrucks.com). if you're on ice, you want anything with studs. little snow with a lotta ice means a/t tires with studs, lotta snow with little ice probably means x/t or m/t tires, studs optional.
-sean