[bekman]

I can't remember the exact figure, but seems like 80-90% of NFL games are decided by outright winner, and the spread is not needed. For this reason, you are better off getting the +payout from the dog and the lesser -line for the fav covering the spread.
Note: I am saying this just as Dallas-Seattle was a spread win ML loss on Dallas (I bought @ +3).
I would be interested in being confirmed or denied on this, as this is just my supposition.