I'd agree with the argument that slow D is more dangerous. Rapid decompression with an inoperative warning system is obvious, you know you need to descend. Most people do. Slow or gradual decompression with an inoperative warning system comes on gradually with no warning, your decision making is slowly affected and eventually you can't make any good decisions. Furthermore, the most dangerous non-pressurized flights I see are the guys from sea level that decide to go and fly XC for hours on end at 12,000 or somewhere thereabouts, same effect, it gradually creeps up on them. The ones that are cruising at 9 or something and go up to 13 to get over a pass and then back down do not seem to experience this nearly as much. Back when I did checkrides I'd occasionally have a person plan an IFR flight at some "legal, but long time at high altitude" flight to comply with some MEA. The question "do you really know what you are doing?" was on the forefront then.