Insulating a narrow depth

Might be worth the extra 1/4" to get another R2.5, I suppose. That I probably will do.

The goal of the air channel beneath the roof is to keep the roof deck cold enough to minimize ice dams from melting the snow from underneath. This requires good airflow. Compromise that air flow, and you might as well insulate it really well. If the space is well sealed, you won't get condensation since there is no air movement to get it there. I'd consider having pros foam it...that way it will fill up all of the nooks and crannies that might be missed when you cut foam panels, or try to stuff fiberglass in there.