The quarterback position is defined by victories on the biggest stage. Dan Marino is never mentioned among the greatest of all time because he never won a Super Bowl. Brady and Manning have both won titles before, so nerves should not be a factor. Both are used to the strains of playing in the big game--media week, the pregame festivities, the elongated halftime, etc. So here is a break down between the two signal callers.
Eli's ability to limit turnovers especially late in the season has been key to the Giants success. If the Giants Offensive line can give Manning time his improved decision making should take over and he could be incredibly dangerous. In the playoffs this year, Eli has a quarterback rating of 103.1. Manning has looked like a completely different player than the one who beat the Patriots in 2008.
For Brady to have success his offensive line is going to have to slow down the Giants pass rush. Given time, Brady is arguably the best at picking apart defenses and making good decisions (Eli's older brother being the other--when healthy). Despite his numbers in the AFC Championship game Brady's QB rating in the playoffs is 105.8.
I give the edge to the Patriots. I think Brady will bounce back from his performance against the Ravens and have another fantastic game on the worlds biggest stage. Additionally, Brady has more weapons to throw to than Manning. The Giants have seen the emergence of Victor Cruz as a deep threat, but Brady has two of the best tight ends in the game along with one of the best slot receivers. With his almost unlimited weapons and superior decision making Brady could alone be the difference in the game.