One striking thing about Marshall's offense is that I don't rate them to have had a bad game either running or passing the ball for the entire season. Not once! That being said, they didn't have any really good offensive days versus any good/top 40 defenses. The good news is that NIU doesn't have a great defense. I currently rank them 91st, but NIU's pass defense was much improved in the second half of the season -- as good of a turn around as any team in CFB, really. Their run defense is still vulnerable, though they had their best game of the season in week 14 against Western Michigan (who are just an average running team).
Defensively, Marshall hasn't faced many good offenses. Western Kentucky shredded Marshall both on the ground and through the air. NIU's passing game isn't anywhere near as good as Western Kentucky's, but NIU's run game is better, so they should test Marshall plenty on the ground.
In the end Marshall has the much more talented offense, but their passing game has not done as well in their last few games. Their defense isn't playing great either, which is why they've either lost outright or failed to cover as double digit opening dogs, whereas earlier in the season (against weaker teams) they were.
I still like Marshall to cover, and have them in a teaser, but am going to pass on making a regular spread play on them, as NIU as a team was heading in a better direction as a team than Marshall was down the stretch.
I bought a couple of units on the opening Over 61.5, but the value is pretty much gone out of the total now at 67. I still think it will clear that number -- it may go way past the number, you never know with two uptempo teams with offensive advantages in the game.