I just listed all of the Giants starting offensive lineman that have performed at least average this season. The Giants have had tons of turnover along the offensive line, losing mainstays Rich Seubert and Shaun O'Hara over the offseason, and losing Will Beatty to an eye problem midway through the season. They have gotten solid production from backups Kevin Boothe and Mitch Petchrus, but overall, the offensive line is probably the team's biggest weakness.
David Diehl struggled mightily at left guard, and has been a liability at left tackle. Kareem McKenzie's best days are well past, and I would be surprised to see him starting again next season. Free agent acquisition David Baas has been a buust, although injuries may be partially responsible. Only Chris Snee was above average, but his season was a down one for his standards (he still made the pro bowl, but that was based more on reputation than actual performance).
The problem is not pass protection; the Giants gave up only 28 sacks all season, although that also had a lot to do with Manning's increased sense of pocket awareness and escape-ability. They were putrid in the NFC Championship game (cut them some slack...wet grass and a dynamite 49ers defense), though, and they are one of the NFL's worst units in terms of run blocking.
The Patriots have only given up one sack in two playoff games, and it's no stretch to say that their battle with the Giants' D-line is the matchup to watch.
Offensive line is the most difficult area to analyze if you have not watched a team consistently over the season and have not broken down game tape. Most of the time, reputation plays a big role in how offensive lineman are perceived (see: Chris Snee making the pro bowl). Simply put, the Patriots' offensive line has a much better reputation than that of the Giants.
Although this unit has also seen massive injuries (starting center Dan Koppen has been on injured reserve for almost the entire season), the elite left side of Matt Light and Logan Mankins basically trumps the Giants' O-line by themselves. Green but immensely talented, rookie Nate Solder holds down right tackle next to veteran pro bowler Brian Waters. Despite all of the mixing and matching, the Patriots' line has done an enviable job of keeping Tom Brady upright and opening modest holes for Benjarvus Green-Ellis and company.
Most importantly: New England's O-line also includes local East Meadow product, and my high school classmate, Rich Ohrnberger (when someone mentioned "Rich," 95% of the time, they weren't talking about me), although he is on the IR. Chad Ochocinco and Rob Gronkowski called Ohrnberger the funniest person on Twitter.
Since I have only seen about eight Patriots games this season, I looked for some deeper statistcal analyzation to help me rate their offensive line. I found a study done by Football Outsiders that ranked each team's offensive line based on run blocking and pass protection. The rankings passed the smell test in how they rated the Giants: #28 in run blocking and #6 in pass protection, which about jives with how I feel the line performed during the regular season (maybe a tad too kind in pass blocking, but not too far off). This same study found that the Patriots rank 2nd in run blocking and 8th in pass protection.
Pro Football Focus also rated the Patriots' offensive line to be far superior to the Giants' in pass blocking; the Patriots were rated as having an 83% pass blocking efficiency (eighth best), while the Giants ranked a shabby 72.6%.
All of these advanced metrics aside, I think most people would agree that the Patriots have the advantage on the offensive line. I give New England the big win for this unit; Patriots: 8.7, Giants 7.5.