VE7DAC
Registered
My AN/DP course (one dive left) has all been air backgas. If you dive the best nitrox mix for a given depth, you won't have a lot of deco to do. Which is normally a good thing, but not so much when you want to get practice with actual deco. Plus air is cheap/free depending on the deal with your shop (I've got a yearly fill card) while O2 has an associated cost.