The tale of the tape in the regular season game between the Giants and Niners doesn't leave much to work with. The team that won (SF) was the team which forced one more turnover. Otherwise, NY threw for more yards, rushed for more yards, converted on 4th down twice...
It's a coin flip. Both defenses are stellar. The Niners' secondary looked awesome to me for much of the game against the Saints, but still had mental lapses and poor tackling in the 4th quarter. The Giants' defense sort of shut down the Packers, but with all the dropped balls by GB receivers it makes it tough to judge.
Whichever team turns the ball over more will lose, which may boil down to whichever QB makes poor decisions or whichever RB has butterfingers, or special teams errors...?