I have the unique perspective of working in the container industry. I actually work as a designer for one of Americase's competitors (on the industrial side, not the gun case side, I can't convince our sales department to make gun cases and give me one).
If you live in a dry climate or don't let your guns sit in the case for long, open cell will be okay. From my perspective, though, if you're selling a gun case, it should be closed cell foam since corrosion is almost always a concern.
I'll point out that some of Americase's cases do have open cell foam. Best to call to make sure, and avoid egg crate.
