If I can do multilevel dive I use EAN21.
If it is a square profile on a new site(24-36m) then I might consider using Nitrox.
If it is multi-dive(>3) in a single day then I will use nitrox on couple of them.
Have to be flexible.
Have you done a cost-benefit analysis? Is that why you switch to air?
What do you see when you run a table? I'm wondering how those air dives work your N2 loading.