Talked to one of my teachers regarding this subject. He admitted that when you're a Paramedic, and you've got All the Airway Toys at your fingertips, he goes from OPA to Intubation...
So I put it into perspective: "If you were packing a paramedic's bag you had to CARRY into the woods camping with you...would you choose to take the NPA's over a full Pharyngeal Airway set?"
He then reconsidered the utility of the NPA in a setting that wasn't so well-equipped, where space and weight were of the essence, and he said, "Ya know, that gives me an idea..."
He considered the OPA surperior for airway managment because of the tongue-depressing action of the OPA...but you can't get around 1) Small size 2) Packability (not rigid) 3) Light weight 4) Adjustibility in the field (cut the bevel of a long tube to length) and 5) Tolerance by both conscious and unconscious patients, all advantages of the NPA in a less-than-optimal field setting.
However...don't forget the lubricant. If you're going to go with NPA's, you need to bring along some Kentucky Jelly to make 'em slide in...
That could be a reason for preference right there. Must remember to ask.
Panz