The Cowboys are a far more talented team than the Lions, across the board. If that's not enough for you, then their ace QB, Matt Stafford, will be replaced by their backup, Jeff Driskel, who isn't anyone to compete with Stafford, although he's certainly more mobile than Stafford ever was. That shouldn't tip the scales in their favor by any stretch, though. They lack an effective running game, as well and their defense is one of the NFL's worst.
If the Cowboys fail to beat the Lions and I mean, beat them impressively, they have no business even being considered as a playoff contender. The level of impetus that the Cowboys have shown vs. winning teams has been far less than satisfactory, although, the Lions aren't winners. We can forget about the Cowboys going anywhere this year, in the playoffs or otherwise, if they fail to play with the necessary resolve in Detroit.