IMO, both defenses are very stout, but Dallas just has so many more weapons with which to score. Jacksonville is without Jimmy Smith and Greg Jones, two big contributors to the 2005 offense. The defense is without Mike Peterson, their leading tackler in 2005 and one of their sack leaders.
As for the 12-5 record last year, notice that the Jags were 3-4 against teams with winning records. The record of opponents in the other 10 games? A combined 43-117. They can only play the teams in front of them, but the Jags played many games against some of the worst teams in the league, including 6 against Hou, Tenn, SF, and the Jets. The 12-5 record was smoke and mirrors, and the Jags were exposed (3-28) by a veteran NE defense in the playoffs.
It's always hard to win against good defenses, especially on the road. I don't see the Jags scoring too many points, so that should give Dallas a good chance to pull this one out.