First, a quick overall comparison. During the regular season the 49ers defense was far superior to the Giants. San Francisco was fourth in the league overall, surrendering 308.2 yards per game. The 49ers allowed just 16.7 points per game, second in the league to the Pittsburgh Steelers, who allowed 16.5. The Giants allowed 376.4 yards per game, 27th in the league, and 25.0 points per game (25th overall).
The Giants have been much better in recent weeks and dominant in the playoffs defensively. The defense did not allow a point in a 24-2 victory over the Atlanta Falcons, and held the high-scoring Green Bay Packers to two touchdowns in a 37-20 victory in the NFC Divisional Round.
Now, let's compare the teams position-by-position.
The Giants play a 4-3 and depend on their front four to pressure the passer and control the line of scrimmage. The 49ers play a 3-4 and largely ask their defensive line to clear the way for their impressive group of linebackers to make plays.
It is no secret that the Giants are built defensively around their front four, especially their pass-rushing defensive. The Giants registered 48 sacks, third in the NFL. Jason Pierre-Paul was named All-Pro after registering 16.5 sacks this season. Osi Umenyiora had nine sacks and two forced fumbles in just nine regular-season games during an injury-riddled season. He has three sacks and a forced fumble in the two playoff games. Defensive end Justin Tuck is also finally healthy.
In addition, the Giants have received solid play all season from tackles Chris Canty, Linval Joseph and Rocky Bernard.
Defensive end Justin Smith is the biggest play-maker on the San Francisco front line. The defensive end registered 7.5 sacks and 58 tackles. Smith made the game-saving play when the 49ers and Giants met in the regular season, batting down an Eli Manning pass on fourth-and-two from the San Francisco 10-yard line to preserve a 27-20 victory.
Advantage: Giants ... The Giants are built around the line and it has been playing well of late.
In the same way the Giants defense is built around defensive lineman, the 49ers defense is built around its linebackers -- and the 49ers have an impressive corps of linebackers. Second-year man Navorro Bowman had a team-high 143 tackles, two sacks and seven passes defensed. Patrick Willis had 97 tackles, 12 passes defensed, four forced fumbles and two sacks. Both players were named All-Pro. Rookie Aldon Smith led the 49ers with 14 sacks. Ahmad Brooks had seven sacks, second on the team behind Smith.
The Giants best linebacker is Michael Boley, who had 93 tackles this season. Chase Blackburn has helped in the middle, Mathias Kiwanuka has excelled against the run and rookie Jacquian Williams has shown flashes and become a reliable pass defender.
Advantage: 49ers ... By a wide margin. The Giants linebackers really aren't a full-fledged weakness, but they can't measure up to San Francisco's.
The Giants have the very solid veteran Corey Webster at one corner, and the sometimes shaky veteran Aaron Ross at the other corner. Rookie Prince Amukamara sees some time as a nickel corner, but safety Antrel Rolle also moves down and guards slot receivers quite often. Webster had 16 passes defensed, six interceptions and a Pro Football Focus score of +4.9.
Carlos Rogers of the 49ers had 18 passes defensed, six interceptions and a Pro Football Focus score of 9.4. Tarell Brown had four interceptions and 15 passes defensed, and rookie Chris Culliver is the nickel back.
Advantage: 49ers ... It seems like a slight advantage, but Rogers-Brown is just a hair better than Webster-Ross.
During the past few weeks, when the Giants defense has played its best football of the season, the team has returned to relying heavily on the three-safety look that was successful for the team in 2010. That means Kenny Phillips, Deon Grant and Rolle all on the field at the same time. None of those players is a star, though Rolle led the Giants in tackles with 96 and Phillips had an excellent +8.6 Pro Football Focus score. The primary issue with the trio is speed, but they have played solidly in recent weeks as the Giants have surged to the NFC title game.
Donte Whitner and Dashon Goldson are the starting safety tandem for San Francisco. Goldson had 67 tackles and Whitner 62 for the 49ers this season. Whitner had a +8.3 Pro Football Focus score, while Goldson was -8.1.
Advantage: Giants ... Grant, Phillips and Rolle form a solid veteran trio. On the field together they don't make many mistakes.
By the season-long numbers the 49ers would appear to have the better defense. Judging by the way the teams have played the last few weeks -- with the Giants finally looking like the pass-rushing, dominant defense they were expected to be -- this really looks like a toss-up.